Sending GitHub notifications to the Mailing List

[simoneonofri]

As discussed with @mirjak, I prepared a PR to send a notification from GitHub to the Mailing List.

w3c/github-notify-ml-config#293

@martinthomson maybe some configuration in the mailing list is needed to allow noreply@w3.org as a sender

[mnot]

Hmm, we have this on the IETF side, may be easier:
https://github.com/ietf-github-services/activity-summary