docs(readme): update link to CONTRIBUTING [skip ci]

Author: myii

docs/README.rst

@@ -65,7 +65,7 @@ Contributing to this repo
 
 **Commit message formatting is significant!!**
 
-Please see :ref:`How to contribute <CONTRIBUTING>` for more details.
+Please see `How to contribute <https://github.com/saltstack-formulas/.github/blob/master/CONTRIBUTING.rst>`_ for more details.
 
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