Documentation for breaking release workflow

[MakisH]

Do we already have a section where we explain the general workflow for a breaking release? > This is not directly related to the schedule but I think it would be helpful to at least point to this workflow from here.

Some points that are important in my opinion:

• What we did last time: At some point there was a v3_develop branch in parallel to the develop branch. At some later point we decided to merge v3_develop into develop. This closed the door for any v2 releases and forced everybody to integrate new features directly with v3.
• I think it would also be helpful to point to the milestone that we usually create in order to cluster breaking releases way before the breaking release is scheduled (see https://github.com/precice/precice/milestone/20).

Originally posted by @BenjaminRodenberg in #475 (comment)