debian/README.source: add documentation about the development process

Since @guido.gunther recently suggested to improve the update process, let's do that and also write it down, including some already established practices.

For this particular updating proces where we want to stabilize a new tree while continuing to accept MRs "to current" as usual, I suggest (and added) the "regression" Gitlab label. Use this for bugs the only happen "on next" and not "on current". All regressions must be resolved before switching next to current (i.e. making a new major version the default). That way, all issues labelled "regression" automatically only point to problems with the latest update process, and are not outdated.

I tried to describe the above in the text doc too and left room to describe a future discussion about Pureos distributions.