Improve patch release checklist
Created by: nicksnyder
A few changes inspired by https://github.com/sourcegraph/sourcegraph/issues/2682
- I liked that @slimsag made a checklist of changes included in the release. This is now documented so others do the same thing in the future.
- It felt silly to have @slimsag block for permission on the release captain (me), so I changed the requirement to be posting to #dev-announce. Posting to #dev-announce is good communication regardless.
- Document how to push a single tag instead of all tags.