Add contributing guidelines

.github/CONTRIBUTING.md

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+# Contributing
+
+Having trouble working with the theme? Found a typo in the documentation? Interested in adding a feature or [fixing a bug](https://github.com/mmistakes/minimal-mistakes/issues)? Then by all means [submit an issue](https://github.com/mmistakes/minimal-mistakes/issues/new) or [pull request](https://help.github.com/articles/using-pull-requests/). If this is your first pull request, it may be helpful to read up on the [GitHub Flow](https://guides.github.com/introduction/flow/) first.
+
+Minimal Mistakes has been designed as a base for you to customize and fit your site's unique needs. Please keep this in mind when requesting features and/or submitting pull requests. If it's not something that most people will use, I probably won't consider it. When in doubt ask. 
+
+This goes for author sidebar links and "share button" additions -- I have no intention of merging in every possibly option, the essentials are there to get you started :smile:.
+
+## Pull Requests
+
+To help me out try to avoid creating pull requests on `master` and instead branch off of `develop`. It's much easier for me to test, merge, and roll them into new releases this way.