Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to P+
+
+:+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+The following is a set of guidelines for contributing to P+. These are mostly guidelines, not rules. Use your best judgment, and feel free to propose changes to this document in a pull request.
+
+### Pull Requests
+* Send your pull requests to the `dev` branch
+* Avoid platform-dependent code
+* Use the present tense ("Add feature" not "Added feature") in the pull request title