Add contribution docs

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# Contributing to Presto
+
+## Contributor License Agreement ("CLA")
+
+In order to accept your pull requests, we need you to submit a CLA. You only need to do this once, so if you've done this for another Facebook open source project, you're good to go. If you are submitting a pull request for the first time, just let us know that you have completed the CLA and we can cross-check with your GitHub username.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## License
+
+By contributing to Presto, you agree that your contributions will be licensed under the [Apache License Version 2.0 (APLv2)](LICENSE).