Merge pull request #4818 from edx/jhelbert/contributing-generalization

Generalizing CONTRIBUTING.rst to all repos

CONTRIBUTING.rst

 ############################
-Contributing to edx-platform
+Contributing to Open edX
 ############################
 
-Contributions to edx-platform are very welcome, and strongly encouraged! We've
+Contributions to Open edX are very welcome, and strongly encouraged! We've
 put together `some documentation that describes our contribution process`_,
 but here's a step-by-step guide that should help you get started.