Mention CLA in PR process

Author: michele-mancioppi

CONTRIBUTING.md

@@ -25,7 +25,8 @@ There are three important things to know:
 
 This a checklist of things to keep in your mind when opening pull requests for this project.
 
-0. Before pushing anything, validate your pull request with `go test`.
+-1. Before pushing anything, validate your pull request with `go test`
+0. Make sure you have signed the [Contributor License Agreement](#contributor-license-agreement)
 1. Make sure any added dependency is licensed under Apache v2.0 license
 2. Strive for very high unit-test coverage and favor testing productive code over mocks
    (mock in depth wherever possible)