slapos: Provide update-hash README

README.update-hash.rst

+update-hash
+===========
+
+``update-hash`` is a development tool to automatically work with
+``buildout.hash.cfg`` files.
+
+Working with ``buildout.hash.cfg``
+----------------------------------
+
+``buildout.hash.cfg`` files are buildout-style simplified configparser files to
+have a easy way to update MD5 hashes of provided files for download. They
+look like::
+
+  [section]
+  md5sum = <hash>
+  filename = <relative-path>
+
+Where ``<hash>`` is an automatically calculated checksum of ``<relative-path>``.
+
+Then ``buildout.hash.cfg`` can be included in software profile by ``extends`` of
+``[buildout]`` section, and the section's ``md5sum`` and ``filename`` can be used.
+
+Special cases of ``filename`` key
+---------------------------------
+
+In case if section recipe has special unwatned behaviour for ``filename`` field
+the ``_update_hash_filename_`` key can be used like::
+
+  [section]
+  md5sum = <hash>
+  _update_hash_filename_ = <relative-path>
+
+Working with ``update-hash``
+----------------------------
+
+In order to update the ``buildout.hash.cfg`` it's enough to call ``update-hash``
+while being in the file directory.