detect new contributors

[Remi-Gau]

It is becoming very hard to keep track of all the new contributors...

TODO: Add a github workflow that checks if a user opening a PR if already listed in the BIDS contributors.

If not add a welcome message to the PR and indicate where they can add themselves as BIDS contributors.

Ideally this workflow should be copy pasteable and work for any repository in case some other BIDS related repo wants to reuse it.