[TRI-661] We need to add instructions in CONTRIBUTING.md for adding changesets to PRs

[ericallam]

null

_TRI-661

[kaf-lamed-beyt]

Since I just learned how to add a changeset. I'd love to do this @ericallam

[kaf-lamed-beyt]

Hi @ericallam, I'm waiting for your review on #200 so I can fix it before starting this, to avoid having the commits from that branch in this one when I start working on it.