Added CONTRIBUTING.md

Author: alibitek

CONTRIBUTING.md

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+Contributing to python-gcm
+===
+We'd love to accept your patches!
+To contribute to python-gcm simply open a pull request against the [develop branch](https://github.com/geeknam/python-gcm/tree/develop)
+
+Some things that will increase the chance that your pull request is accepted:
+* Succinctly describe your changes in the pull request description
+* Write tests:
+  * new features should include tests
+  * changes to existing code should be reflected in the tests
+  * code coverage should not decrease
+  * run **all** the tests to assure nothing else was accidentally broken.
+* Be consistent with the coding style
+* Make commits of logical units
+* Write descriptive commit messages
+* Check for unnecessary whitespace with `git diff --check` before committing.