[451] Updated CONTRIBUTING.md
Some basic changes to CONTRIBUTING.md.
Adds more of the missing labels and uses version/milestone branches instead of the staging
branch as the target for merge requests.
Unfortunately, my Git-fu isn't that great so this also includes the !257 (merged) merge request and recent hotfixes that aren't in the v0.1.10
branch, but I'm sure there's a way to remove them if they aren't wanted.