Add or later to the license expressed in the README

This is a clarification, not a relicensing.

Our source code says "GPLv3+" or "version 3 of the License, or later".
Our documentation says GPLv3+:

https://github.com/ansible/ansible/blob/devel/docs/docsite/rst/dev_guide/developing_modules_checklist.rst#contributing-to-ansible-objective-requirements

We were just lazy when we wrote the README and left out the "or later".
this update to the README brings it in line with what everything else
says.

README.rst

@@ -84,7 +84,7 @@ and has contributions from over 4700 users (and growing). Thanks everyone!
 License
 =======
 
-GNU General Public License v3.0
+GNU General Public License v3.0 or later
 
 See `COPYING <COPYING>`_ to see the full text.