Updating the handbook to remind people to tag if appropriate

We can tag people in MRs and Issues, but we don't always do it thoughtfully. If you want to make people aware, or a group of people, you tab them, but should also clearly identify why they are being tagged. example FYI, FYA? If additional context is useful, add that too. As we grow, the number of changes grow and this will help everyone manage what they need to take action on a bit better, but also hopefully keep people informed.

FYI: @gl-managers