Add description of RFC process

README.rst:
Add a description about what the RFC process means and how it works in
the context of using Gitlab with merge requests to assign RFC numbers
automatically.
Provide examples for announcements and describe the workflow of how to
propose, comment on and finalize an RFC.
Add a section about who can file an RFC and under what circumstances
outside contributors are able to file an RFC (only if at least one
Developer or Trusted User supports the effort).
Add a note for how to create a merge request (i.e. allowing commits from
members who can merge to the target branch).
Add note for users without access rights to the arch-dev-public mailing
list.
Add link to creation of a new ticket for the repository as a way of
proposing an idea for an RFC.
Add a note with link to the repository issues for current or previous
ideas (in ticket form).
Mention GitLab's approval feature as a means to approve of an RFC in the
Final Comment Period.
Add section about licensing the contributions under the terms of the
CC-BY-SA 4.0 license.