Fix broken link, rename CONTRIBUTING to CONTRIBUTING.md

Summary:
Before:
There was a broken link in ./docs/README.md
CONTRIBUTING didn't render markup on Github

After:
All links to CONTRIBUTING.md are not broken anymore
CONTRIBUTING.md renders markup on Github
Closes https://github.com/facebook/yoga/pull/458

Differential Revision: D4673746

Pulled By: emilsjolander

fbshipit-source-id: 151c1f38789a96b61dfbb544fd11996098dc456d

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+# Contributing to yoga
+We want to make contributing to this project as easy and transparent as
+possible.
+
+## Pull Requests
+We actively welcome your pull requests.
+1. Fork the repo and create your branch from `master`.
+2. If you've added code that should be tested, add tests
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints.
+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
+* format.sh
+
+## License
+By contributing to yoga, you agree that your contributions will be licensed
+under its BSD license.