Add contribution guide

CONTRIBUTING.md

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+## Contributing
+
+This is a pretty fun, non-serious hacking project, so if you're interested in
+contributing, sign up, clone the project, and submit some merge requests!
+There's a lot to add and work on, so join in.
+
+## Code Style
+
+4 spaces per indent level. 120 character line length. Follow PEP8 as closely
+as reasonable. There's a prospector config, use it.