Release branch

[joshua-white]

The code is getting mature enough we should start planning ahead for a more complicated Git model, with separate release and develop branches. Our issue keeping the tutorials up to date is a good example/motivator. I don't think we need to do this today, but we should think a bit about how we want to handle release cycles going forward. Thoughts?

[rrsettgast]

We can just go back to the gitflow model...or just a straight OneFlow...or we can actually follow through with the modified OneFlow that I had proposed be our model.

[paveltomin]

Seems already implemented with tags https://github.com/GEOS-DEV/GEOS/tags ?