Add a CLA

[fulmicoton]

https://github.com/cla-assistant/cla-assistant offers a project with a github action.

We can either host it, use the SAP hosted version, or use cla-assistant-lite which is alpha, and host the data directly in the repo.
The latter is convenient but a bit scary and does not allow for a individual/corporate contribute license agreement.

For the template itself, we can probably rely on http://www.harmonyagreements.org/faqs.html

We also need to document the CLAs and the process in a contributing.md file.