Add instructions for contributing.

README.md

@@ -9,3 +9,20 @@ go get github.com/golang/oauth2
 ~~~~
 
 See [godoc](http://godoc.org/github.com/golang/oauth2) for further documentation and examples.
+
+## Contributing
+
+Fork the repo, make changes, run the tests and open a pull request.
+
+Before we can accept any pull requests
+we have to jump through a couple of legal hurdles,
+primarily a Contributor License Agreement (CLA):
+
+- **If you are an individual writing original source code**
+  and you're sure you own the intellectual property,
+  then you'll need to sign an [individual CLA](http://code.google.com/legal/individual-cla-v1.0.html).
+- **If you work for a company that wants to allow you to contribute your work**,
+  then you'll need to sign a [corporate CLA](http://code.google.com/legal/corporate-cla-v1.0.html).
+
+You can sign these electronically (just scroll to the bottom).
+After that, we'll be able to accept your pull requests.