Update CONTRIBUTING.md

Author: otw

CONTRIBUTING.md

@@ -13,6 +13,7 @@ Please fill out either the individual or corporate Contributor License Agreement
   * If you work for a company that wants to allow you to contribute your work,
     then you'll need to sign a [corporate CLA]
     (https://cla.developers.google.com).
+  * Please make sure you sign both, Android and Google CLA
 
 Follow either of the two links above to access the appropriate CLA and
 instructions for how to sign and return it. Once we receive it, we'll be able to