Add hint on adding VERSION to distributions.

Author: carljm

source/single_source_version.rst

@@ -84,7 +84,8 @@ There are a few techniques to store the version in your project code without dup
     .. warning::
 
         With this approach you must make sure that the ``VERSION`` file is included in
-        all your source and binary distributions.
+        all your source and binary distributions (e.g. add ``include VERSION`` to your
+        ``MANIFEST.in``).
 
 #.  Set the value in ``setup.py``, and have the project code use the
     ``pkg_resources`` API.