Follow all-contributors specification in README.md

[meliache]

See the all-contributors specification on github.

We put all (even single-commit) contributors into our the team section on the sphinx documentation anyway, but having them in the README seems to be more conventional and might offer additional visibility for contributors.

The all-contributors project even comes with a bot for adding contributors. Today I contributed to scikit-hep hist and got added by their contributors bot, which is how I learned about that specification.

It would be nice if we could somehow generate the sphinx contributors docs from the same source, otherwise we just might shorten that and add link to the readme for that.