Add CONTRIBUTING

CONTRIBUTING.md

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+If you want to suggest a change or addition that will help accomplish
+[the goals](README.md) of this style guide, please open a pull request that:
+
+ 1. Explains the guideline
+ 1. Demonstrates the guideline with more-or-less valid example code
+ 1. Justifies the guideline by explaining the rationale behind it
+
+Just note that all suggestions are open to discussion and debate! :smile: