Update CONTRIBUTING.md

Use CLA assistant for signing ICLA.

Signed-off-by: Ivan Sokolov <[email protected]>

CONTRIBUTING.md

@@ -6,3 +6,10 @@ When adding a new feature or modifying an existing one please ensure that:
 - It is covered with unit tests
 - Module documentation is updated accordingly
 - Demo data is added or updated
+
+By submitting a pull request or otherwise contributing to this project, you agree to the terms of our CLA.
+We are using [CLA Assistant](https://cla-assistant.io/) to automate the CLA signing process.
+
+If you are contributing as an individual there will be an invitation posted in your first PR. You will need to follow the link, review the CLA and sign it.
+
+If you are contributing as a legal entity you need to contact [Cetmix OÜ](https://cetmix.com) directly.