Contributor License Agreement (CLA) workflow

[hanneshapke]

Regarding the CLA workflow:

If we can use a Gist on Github, a repo admin could set up the CLA from https://cla-assistant.io/ (built by SAP).

The setup looks like below (just with the correct repo and Gist)

An alternative is this PR:
It uses https://github.com/cla-assistant/cla-assistant but runs the entire signing without the need for Gist. The text can be loaded from the repo. Downside: 1) It needs a write token to write the user information to a file, and 2) the file with the CLA "signature information" is part of the repo (Name and email are visible through their commits anyway) (admin needed). We could also write to a private repo that only contains the CLA signature information, but we would need to set up another repository (admin needed).

@pbertin Which option do you prefer?

1. External service (admin needed for the setup)
2. This PR and the information are stored in this repo (admin needed for the token creation)
3. This PR with the data been pushed to a separate, private repository (admin needed for the token and repo creation)