doc: review process: clarification regarding git(hub) conventions

This is also demo pull request according to the proposed changes.
I am not sure myself about 5925c9c, though.

This ends the formal introduction (in case the ticket would be a few days old), by convention I could include a proposed git log message, but the single commit are named to work well in the auto generated message of a squash-merge.

---

- demand pull requests
- use "Signed-off-by"
- 'relaxed core' has to self-sign