This shows up for both pull requests and creating new issues; make this more...

This shows up for both pull requests and creating new issues; make this more clear that the CLA only applies to pull requests if you come from the issue page.

CONTRIBUTING.md

 ## Contributing to Pyston
 
-To contribute to Pyston, you need to to sign the [Dropbox Contributor License Agreement](https://opensource.dropbox.com/cla/), if you haven't already.
+#### Pull Requests
+
+Before a pull request can be merged, you need to to sign the [Dropbox Contributor License Agreement](https://opensource.dropbox.com/cla/).
 
 Please make sure to run the tests (`make test`) on your changes.