Add contributing guide

CONTRIBUTING.md

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+Contributing
+============
+
+All kinds of contributions to Quill are greatly appreciated. For someone
+unfamiliar with the code base, the most efficient way to contribute is usually
+to submit a [feature request](#feature-requests) or [bug report](#bug-reports).
+
+If you want to dive into the source code, you can submit a [patch](#patches) as
+well. Working on [existing issues][issues] is super helpful!
+
+Feature Requests
+----------------
+
+Do you have an idea for an awesome new feature for Quill? Please [submit a
+feature request][issue]. It's great to hear about new ideas.
+
+If you are inclined to do so, you're welcome to [fork][fork] Quill, work on
+implementing the feature yourself, and submit a patch. In this case, it's
+*highly recommended* that you first [open an issue][issue] describing your
+enhancement to get early feedback on the new feature that you are implementing.
+This will help avoid wasted efforts and ensure that your work is incorporated
+into the code base.
+
+Bug Reports
+-----------
+
+Did something go wrong with Quill? Sorry about that! Bug reports are greatly
+appreciated!
+
+When you [submit a bug report][issue], please include relevant information such
+as Quill version, operating system, configuration, error messages, and steps to
+reproduce the bug. The more details you can include, the easier it is to find
+and fix the bug.
+
+Patches
+-------
+
+Want to hack on Quill? Awesome!
+
+If there are [open issues][issues], you're more than welcome to work on those -
+this is probably the best way to contribute to Quill. If you have your own
+ideas, that's great too! In that case, before working on substantial changes to
+the code base, it is *highly recommended* that you first [open an issue][issue]
+describing what you intend to work on.
+
+**Patches are generally submitted as pull requests.** Patches are also
+[accepted over email][email].
+
+Any changes to the code base should follow the style and coding conventions
+used in the rest of the project. The version history should be clean, and
+commit messages should be descriptive and [properly formatted][commit-messages].
+
+---
+
+If you have any questions about anything, feel free to [ask][email]!
+
+*Thanks to Anish Athalye for allowing Quill to shamelessly steal this contributing guide from [Gavel][gavel]!*
+
+[issue]: https://github.com/techx/quill/issues/new
+[issues]: https://github.com/techx/quill/issues
+[fork]: https://github.com/techx/quill/fork
+[email]: mailto:[email protected]
+[commit-messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[gavel]: https://github.com/anishathalye/gavel