Add CONTRIBUTING.md

The lack of this could explain the wild variation in styles we have seen from our recent contributions.

Author: boidanny

CONTRIBUTING.md

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+We love that you're interested in contributing to this project!
+
+To make the process as painless as possible, we have just a couple of guidelines
+that should make life easier for everyone involved.
+
+## Prefer Pull Requests
+
+If you know exactly how to implement the feature being suggested or fix the bug
+being reported, please open a pull request instead of an issue. Pull requests are easier than
+patches or inline code blocks for discussing and merging the changes.
+
+If you can't make the change yourself, please open an issue after making sure
+that one isn't already logged.
+
+## Contributing Code
+
+Fork this repository, make it awesomer (preferably in a branch named for the
+topic), send a pull request!
+
+All code contributions should match our [coding
+conventions](https://github.com/github/objective-c-conventions).
+
+Thanks for contributing! :boom::camel: