docs: dedicate a file for development mode

docs/userguide/development_mode.txt

+"Development Mode"
+==================
+
+Under normal circumstances, the ``distutils`` assume that you are going to
+build a distribution of your project, not use it in its "raw" or "unbuilt"
+form.  If you were to use the ``distutils`` that way, you would have to rebuild
+and reinstall your project every time you made a change to it during
+development.
+
+Another problem that sometimes comes up with the ``distutils`` is that you may
+need to do development on two related projects at the same time.  You may need
+to put both projects' packages in the same directory to run them, but need to
+keep them separate for revision control purposes.  How can you do this?
+
+Setuptools allows you to deploy your projects for use in a common directory or
+staging area, but without copying any files.  Thus, you can edit each project's
+code in its checkout directory, and only need to run build commands when you
+change a project's C extensions or similarly compiled files.  You can even
+deploy a project into another project's checkout directory, if that's your
+preferred way of working (as opposed to using a common independent staging area
+or the site-packages directory).
+
+To do this, use the ``setup.py develop`` command.  It works very similarly to
+``setup.py install``, except that it doesn't actually install anything.
+Instead, it creates a special ``.egg-link`` file in the deployment directory,
+that links to your project's source code.  And, if your deployment directory is
+Python's ``site-packages`` directory, it will also update the
+``easy-install.pth`` file to include your project's source code, thereby making
+it available on ``sys.path`` for all programs using that Python installation.
+
+If you have enabled the ``use_2to3`` flag, then of course the ``.egg-link``
+will not link directly to your source code when run under Python 3, since
+that source code would be made for Python 2 and not work under Python 3.
+Instead the ``setup.py develop`` will build Python 3 code under the ``build``
+directory, and link there. This means that after doing code changes you will
+have to run ``setup.py build`` before these changes are picked up by your
+Python 3 installation.
+
+In addition, the ``develop`` command creates wrapper scripts in the target
+script directory that will run your in-development scripts after ensuring that
+all your ``install_requires`` packages are available on ``sys.path``.
+
+You can deploy the same project to multiple staging areas, e.g. if you have
+multiple projects on the same machine that are sharing the same project you're
+doing development work.
+
+When you're done with a given development task, you can remove the project
+source from a staging area using ``setup.py develop --uninstall``, specifying
+the desired staging area if it's not the default.
+
+There are several options to control the precise behavior of the ``develop``
+command; see the section on the `develop`_ command below for more details.
+
+Note that you can also apply setuptools commands to non-setuptools projects,
+using commands like this::
+
+   python -c "import setuptools; with open('setup.py') as f: exec(compile(f.read(), 'setup.py', 'exec'))" develop
+
+That is, you can simply list the normal setup commands and options following
+the quoted part.
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