Add note on who does what in a MR

Our flows are written always assuming that the person reading is doing the work the flow describes. However they don't mention who should usually do what. Add some guidance on:

• Who generally opens MRs "closing" an issue. (The person who the issue is assigned to.)
• Who reviews a MR. (Can be many people. Usually not the author.)
• Who tests a MR. (Often the reviewer, can be others. Usually not the author.)