Add CLA info to Contributing doc.

CONTRIBUTING.md

@@ -1,6 +1,30 @@
 # Contributing to Vitess
 
-For external contributors, we recommend the standard [GitHub flow](https://guides.github.com/introduction/flow/)
+## Before you contribute
+
+### Individual contributor
+
+Before we can use your code, you must sign the
+[Google Individual Contributor License Agreement]
+(https://cla.developers.google.com/about/google-individual)
+(CLA), which you can do online. The CLA is necessary mainly because you own the
+copyright to your changes, even after your contribution becomes part of our
+codebase, so we need your permission to use and distribute your code. We also
+need to be sure of various other things—for instance that you'll tell us if you
+know that your code infringes on other people's patents. You don't have to sign
+the CLA until after you've submitted your code for review and a member has
+approved it, but you must do it before we can put your code into our codebase.
+
+### Corporate contributor
+
+Contributions made by corporations are covered by a different agreement than
+the one above, the
+[Software Grant and Corporate Contributor License Agreement]
+(https://cla.developers.google.com/about/google-corporate).
+
+## Workflow
+
+For all contributors, we recommend the standard [GitHub flow](https://guides.github.com/introduction/flow/)
 based on [forking and pull requests](https://guides.github.com/activities/forking/).
 
 For significant changes, please [create an issue](https://github.com/youtube/vitess/issues)