Introduce Contribution License Agreement (CLA) (#628)

* Include CLA in Contribution Guide

* Add CLA Assistant to Thank You

.github/CONTRIBUTING.md

@@ -6,6 +6,7 @@ This guide provides guidelines on how to contribute, get involved or report feat
 
 In this guide you can find some guidance on where you can help:
 
+- [Where Can I Help?](#where-can-i-help)
 - [Where Can I Help?](#where-can-i-help)
 - [Pull Requests](#pull-requests)
 - [Requesting A Feature](#requesting-a-feature)
@@ -17,6 +18,14 @@ In this guide you can find some guidance on where you can help:
 
 ## Where Can I Help?
 
+Promitor uses a [Contribution License Agreement (CLA)](https://gist.github.com/tomkerkhove/49506ad58e30076518609511619dc026) which needs to be signed for all contributions and is handled by [CLA Assistant](https://cla-assistant.io).
+
+The Contribution License Agreement (CLA) is required for all parties to align on the expectations of the contributions in order to avoid disputes in the future.
+
+The CLA Assistant will walk you through the process during your first pull request.
+
+## Where Can I Help?
+
 Unsure where to begin contributing to Promitor? You can start by looking through these `beginner` and `help-wanted` issues:
 
 - [Beginner issues](https://github.com/tomkerkhove/promitor/issues?q=is%3Aopen+is%3Aissue+label%3Abeginner) - issues are very small and good to learn how Promitor works.

docs/thank-you.md

@@ -14,6 +14,10 @@ Here is an overview of the services that are so kind to support us:
 
 ![Azure Pipelines](./media/logos/azure-pipelines.png)
 
+- **CLA Assistant** automates the process of signing our Contribution License Agreement (CLA).
+
+![CLA Assistant](./media/logos/cla-assistant.png)
+
 - **CloudFlare** serves our documentation super fast via their fast global Content Delivery Network.
 
 ![CloudFlare](./media/logos/cloudflare.png)