update/add link to corporate agreement, per guidelines

CONTRIBUTING.md

@@ -4,7 +4,7 @@ at the end).
 ### Before you contribute
 Before we can use your code, you must sign the
 [Google Individual Contributor License
-Agreement](https://developers.google.com/open-source/cla/individual?csw=1)
+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
@@ -23,5 +23,5 @@ We use Github pull requests for this purpose.
 
 ### The small print
 Contributions made by corporations are covered by a different agreement than
-the one above, the Software Grant and Corporate Contributor License
-Agreement.
+the one above, the [Software Grant and Corporate Contributor License
+Agreement](https://cla.developers.google.com/about/google-corporate).