[Debian-salsa-ci] Salsa CI documentation updated

[Otto Kekäläinen]

Hi!

We've overhauled the README.md at
https://salsa.debian.org/salsa-ci-team/pipeline to be as complete as
possible, yet clear and to the point. If you are not yet using Salsa
CI for pre-upload quality assurance for your package, you might want
to check out what Salsa CI offers, or just review that your current
usage is optimal.

We also added a RUNNERS.md for those who want to experiment with
customer runners or donate CI capacity to Debian.

Feedback and improvement suggestions are also welcome as Merge
Requests are via email.

- Otto