Add Initial contributing doc.

CONTRIBUTING.md

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+# Contributing
+
+We welcome pull requests from everyone. By contributing to this project, you agree to abide by the Fungus [code of conduct]. You also agree that by submitting a pull request for this project, *your contribution will be licensed under the MIT license for this project*.
+
+Fork and clone the Fungus repo (see [fork a repo] )
+
+Make sure the tests pass locally (more information on how to do this coming soon).
+
+Make your change. Add tests for your change. Make the tests pass locally.
+
+Push to your fork and [submit a pull request][pr].
+
+[pr]: https://github.com/fungusgames/Fungus/compare/
+
+We will try to give initial feedback to pull requests within 3 days or sooner. However, this may not always be possible. Please be patient, we will look at every pull request as soon as we can.
+
+Your pull request will have a better chance of being accepted if you do the following: 
+
+* Write tests (more information on how to do this coming soon).
+* Follow our coding style (details coming soon !)
+* Write a [good commit message][commit].
+
+[code of conduct]: https://fungusgames.com/code-of-conduct
+[commit]: http://chris.beams.io/posts/git-commit/
+[fork a repo]: https://help.github.com/articles/fork-a-repo/