Add a CONTRIBUTING file

Summary: Instructions for submitting pull requests.

Reviewed By: jteller, linuxfood

Differential Revision: D4253064

fbshipit-source-id: 2188a5bfbf172cec2537c8402409a3542e3c2156

CONTRIBUTING.md

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+# Contributing to Beringei
+We want to make contributing to Beringei as easy and transparent as
+possible.
+
+## Our Development Process
+We primarily develop Beringei on an internal branch at Facebook, with commits
+continuously synced to GitHub. Pull requests are rebased onto this internal
+branch and then synced back out.
+
+## Pull Requests
+We actively welcome your pull requests.
+
+1. Fork the repo and create your branch from `master`.
+2. Make sure all changes have appropriate tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure `make test` passes.
+5. clang-format the repo by running `beringei/clang-format.sh`
+6. If you haven't already, complete the Contributor License Agreement ("CLA").
+
+## Contributor License Agreement ("CLA")
+In order to accept your pull request, we need you to submit a CLA. You only need
+to do this once to work on any of Facebook's open source projects.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## Issues
+We use GitHub issues to track public bugs. Please ensure your description is
+clear and has sufficient instructions to be able to reproduce the issue.
+
+Facebook has a [bounty program](https://www.facebook.com/whitehat/) for the safe
+disclosure of security bugs. In those cases, please go through the process
+outlined on that page and do not file a public issue.
+
+## Coding Style
+Coding style is handled entirely by
+[clang-format](http://clang.llvm.org/docs/ClangFormat.html). Please make sure
+to run it against all code changes.
+
+You will need clang-format >=3.9.
+
+The provided script `clang-format.sh` is an easy way to reformat the entire
+repository.
+
+## License
+By contributing to Beringei, you agree that your contributions will be licensed
+under the LICENSE file in the root directory of this source tree.