Document expected merge request work-flow

Seeing that moving to GitLab makes merge requests (MR) a lot more accessible, we need some documentation as to the kind of work-flow(s) that we expect for them. At present, I see the following scenarios:

• project member wants to make changes to code actively maintained by self
• project member wants to make changes to code maintained by others
• project member wants to make changes to code that affects code maintained by others (e.g. sanei/ or the build system)
• external requests to merge changes
  • via a fork on GitLab
  • via clones hosted elsewhere, e.g. GitHub
  • via patches

@llagendijk You mentioned this on the mailing list.