Add a Contributor License Agreement workflow

[Daniel-Mietchen]

i.e. set up a workflow that make sure project contributors have consented to our licensing terms (cf. #934 ).

An example for such an agreement sits at
https://cla-assistant.io/grafana/grafana
and is integrated with the pull request workflow.

[WolfgangFahl]

How about stating that accepting an invitation to the project means accepting the collaboration terms https://github.com/WDscholia/scholia/blob/master/CONTRIBUTING.rst?

[Daniel-Mietchen]

Here is Grafana's CLA: https://github.com/grafana/grafana/blob/main/docs/sources/developers/cla.md

[fnielsen]

Here is another one: https://gist.github.com/InnovativeInventor/2b6dddf5cb79eaa58d3f63b5933abdfd

[fnielsen]

Developer Certificate of Origin https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin