add allcontributors integration to add them all to README.md

[yarikoptic]

instigated by #363

We might even want to adopt "our" https://github.com/con/tributors which we ATM use within https://github.com/datalad/datalad/blob/maint/.github/workflows/update-contributors.yml to fully automate.

It is somewhat blocked by #358 (comment) in that we might better first strip README.md from all documentation which should go into docs/ so might better come later but could be done even now.

[dbkeator]

I agree, would be great to include all contributors and automate this