Add note about git describe

Author: mehulkar

README.rst

@@ -123,6 +123,9 @@ and uses roughly the following logic to render the version:
 The next version is calculated by adding ``1`` to the last numeric component
 of the tag.
 
+For git projects, the version is version is relies on `git describe <https://git-scm.com/docs/git-describe>`_,
+so you will see an additional ``g`` prepended to the ``{revision hash}``.
+
 Semantic Versioning (SemVer)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~