doc: add explanation about CLA in CONTRIBUTING.md (#483)

CONTRIBUTING.md

@@ -13,7 +13,12 @@ discussions.
 Once the proposal is approved, a Pull Request can be opened. If you want
 to provide early visibility to reviewers, create a [Draft Pull Request].
 
+We will also require you to sign the [Containous Contributor License Agreement]
+after you submit your first pull request to this project. The link to sign the
+agreement will be presented to you in the web interface of the pull request.
+
 [Issues]: https://github.com/containous/yaegi/issues
 [Pull Requests]: https://github.com/containous/yaegi/issues
 [Feature Request]: https://github.com/containous/yaegi/issues/new?template=feature_request.md
 [Draft Pull Request]: https://github.blog/2019-02-14-introducing-draft-pull-requests/
+[Containous Contributor License Agreement]: https://cla-assistant.io/containous/yaegi