Add licensing information to contributing document

CONTRIBUTING.md

@@ -127,3 +127,9 @@ When adding a new route to an existing handler use the following as a template (
 ```
 
 explanation about each line can be found (here)[https://github.com/swaggo/swag#api-operation]
+
+## Licensing
+
+See the [LICENSE](https://github.com/portainer/portainer/blob/develop/LICENSE) file for our project's licensing. We will ask you to confirm the licensing of your contribution.
+
+We may ask you to sign a [Contributor License Agreement (CLA)](http://en.wikipedia.org/wiki/Contributor_License_Agreement) for larger changes.