Add CONTRIBUTING.md;

CONTRIBUTING.md

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+Contributing
+===
+
+You want to contribute to LÖVR?  That's awesome!
+
+Submitting Issues
+---
+
+Feel free to file an issue if you notice a bug.  Make sure you search before filing an issue, it may
+have already been asked!
+
+Issues are okay for feature requests and questions about the development of LÖVR as well, but
+usually you'll get a better response by asking in [Slack]().  Quesetions about using LÖVR belong
+in Slack.
+
+Contributing Documentation
+---
+
+If you see any typos or inconsistencies in the docs, submitting an issue or pull request in the
+[lovr-docs repo](https://github.com/bjornbytes/lovr-docs) would be greatly appreciated!  There,
+each documentation page is a markdown file in the `docs` folder, and examples can be found in the
+`examples` folder.
+
+Contributing Code
+---
+
+To contribute patches, fork LÖVR, commit to a branch, and submit a pull request.  For larger
+changes, it can be a good idea to engage in initial discussion via issues or Slack before
+submitting.  Try to stick to the existing coding style:
+
+- 2 space indentation.
+- 100 character wrapping (ish, sometimes it's more readable to just have a long line).
+- Use descriptive, camelCased names when possible.
+
+If you modify the embedded `boot.lua` script, you can compile it into a C header by doing this:
+
+```sh
+pushd src/data; xxd -i boot.lua > boot.lua.h; popd
+```