docs: dedicate a file for datafiles support

docs/userguide/datafiles.txt

+====================
+Data Files Support
+====================
+
+The distutils have traditionally allowed installation of "data files", which
+are placed in a platform-specific location.  However, the most common use case
+for data files distributed with a package is for use *by* the package, usually
+by including the data files in the package directory.
+
+Setuptools offers three ways to specify data files to be included in your
+packages.  First, you can simply use the ``include_package_data`` keyword,
+e.g.::
+
+    from setuptools import setup, find_packages
+    setup(
+        ...
+        include_package_data=True
+    )
+
+This tells setuptools to install any data files it finds in your packages.
+The data files must be specified via the distutils' ``MANIFEST.in`` file.
+(They can also be tracked by a revision control system, using an appropriate
+plugin.  See the section below on `Adding Support for Revision Control
+Systems`_ for information on how to write such plugins.)
+
+If you want finer-grained control over what files are included (for example,
+if you have documentation files in your package directories and want to exclude
+them from installation), then you can also use the ``package_data`` keyword,
+e.g.::
+
+    from setuptools import setup, find_packages
+    setup(
+        ...
+        package_data={
+            # If any package contains *.txt or *.rst files, include them:
+            "": ["*.txt", "*.rst"],
+            # And include any *.msg files found in the "hello" package, too:
+            "hello": ["*.msg"],
+        }
+    )
+
+The ``package_data`` argument is a dictionary that maps from package names to
+lists of glob patterns.  The globs may include subdirectory names, if the data
+files are contained in a subdirectory of the package.  For example, if the
+package tree looks like this::
+
+    setup.py
+    src/
+        mypkg/
+            __init__.py
+            mypkg.txt
+            data/
+                somefile.dat
+                otherdata.dat
+
+The setuptools setup file might look like this::
+
+    from setuptools import setup, find_packages
+    setup(
+        ...
+        packages=find_packages("src"),  # include all packages under src
+        package_dir={"": "src"},   # tell distutils packages are under src
+
+        package_data={
+            # If any package contains *.txt files, include them:
+            "": ["*.txt"],
+            # And include any *.dat files found in the "data" subdirectory
+            # of the "mypkg" package, also:
+            "mypkg": ["data/*.dat"],
+        }
+    )
+
+Notice that if you list patterns in ``package_data`` under the empty string,
+these patterns are used to find files in every package, even ones that also
+have their own patterns listed.  Thus, in the above example, the ``mypkg.txt``
+file gets included even though it's not listed in the patterns for ``mypkg``.
+
+Also notice that if you use paths, you *must* use a forward slash (``/``) as
+the path separator, even if you are on Windows.  Setuptools automatically
+converts slashes to appropriate platform-specific separators at build time.
+
+If datafiles are contained in a subdirectory of a package that isn't a package
+itself (no ``__init__.py``), then the subdirectory names (or ``*``) are required
+in the ``package_data`` argument (as shown above with ``"data/*.dat"``).
+
+When building an ``sdist``, the datafiles are also drawn from the
+``package_name.egg-info/SOURCES.txt`` file, so make sure that this is removed if
+the ``setup.py`` ``package_data`` list is updated before calling ``setup.py``.
+
+(Note: although the ``package_data`` argument was previously only available in
+``setuptools``, it was also added to the Python ``distutils`` package as of
+Python 2.4; there is `some documentation for the feature`__ available on the
+python.org website.  If using the setuptools-specific ``include_package_data``
+argument, files specified by ``package_data`` will *not* be automatically
+added to the manifest unless they are listed in the MANIFEST.in file.)
+
+__ https://docs.python.org/3/distutils/setupscript.html#installing-package-data
+
+Sometimes, the ``include_package_data`` or ``package_data`` options alone
+aren't sufficient to precisely define what files you want included.  For
+example, you may want to include package README files in your revision control
+system and source distributions, but exclude them from being installed.  So,
+setuptools offers an ``exclude_package_data`` option as well, that allows you
+to do things like this::
+
+    from setuptools import setup, find_packages
+    setup(
+        ...
+        packages=find_packages("src"),  # include all packages under src
+        package_dir={"": "src"},   # tell distutils packages are under src
+
+        include_package_data=True,    # include everything in source control
+
+        # ...but exclude README.txt from all packages
+        exclude_package_data={"": ["README.txt"]},
+    )
+
+The ``exclude_package_data`` option is a dictionary mapping package names to
+lists of wildcard patterns, just like the ``package_data`` option.  And, just
+as with that option, a key of ``""`` will apply the given pattern(s) to all
+packages.  However, any files that match these patterns will be *excluded*
+from installation, even if they were listed in ``package_data`` or were
+included as a result of using ``include_package_data``.
+
+In summary, the three options allow you to:
+
+``include_package_data``
+    Accept all data files and directories matched by ``MANIFEST.in``.
+
+``package_data``
+    Specify additional patterns to match files that may or may
+    not be matched by ``MANIFEST.in`` or found in source control.
+
+``exclude_package_data``
+    Specify patterns for data files and directories that should *not* be
+    included when a package is installed, even if they would otherwise have
+    been included due to the use of the preceding options.
+
+NOTE: Due to the way the distutils build process works, a data file that you
+include in your project and then stop including may be "orphaned" in your
+project's build directories, requiring you to run ``setup.py clean --all`` to
+fully remove them.  This may also be important for your users and contributors
+if they track intermediate revisions of your project using Subversion; be sure
+to let them know when you make changes that remove files from inclusion so they
+can run ``setup.py clean --all``.
+
+Accessing Data Files at Runtime
+-------------------------------
+
+Typically, existing programs manipulate a package's ``__file__`` attribute in
+order to find the location of data files.  However, this manipulation isn't
+compatible with PEP 302-based import hooks, including importing from zip files
+and Python Eggs.  It is strongly recommended that, if you are using data files,
+you should use the :ref:`ResourceManager API` of ``pkg_resources`` to access
+them.  The ``pkg_resources`` module is distributed as part of setuptools, so if
+you're using setuptools to distribute your package, there is no reason not to
+use its resource management API.  See also `Importlib Resources`_ for
+a quick example of converting code that uses ``__file__`` to use
+``pkg_resources`` instead.
+
+.. _Importlib Resources: https://docs.python.org/3/library/importlib.html#module-importlib.resources
+
+
+Non-Package Data Files
+----------------------
+
+Historically, ``setuptools`` by way of ``easy_install`` would encapsulate data
+files from the distribution into the egg (see `the old docs
+<https://github.com/pypa/setuptools/blob/52aacd5b276fedd6849c3a648a0014f5da563e93/docs/setuptools.txt#L970-L1001>`_). As eggs are deprecated and pip-based installs
+fall back to the platform-specific location for installing data files, there is
+no supported facility to reliably retrieve these resources.
+
+Instead, the PyPA recommends that any data files you wish to be accessible at
+run time be included in the package.
\ No newline at end of file