meta: Proposals should link back to the main PRs that make them implemented?

Citing stuff in https://github.com/osrf/sdformat/pull/275 was kinda painful.
It would be nice if this were more explicitly discoverable, rather than having to sleuth.

Ideal outcome:
The current [`proposal_format.md`](https://github.com/osrf/sdf_tutorials/blob/9b426e9a25ae478a76f31f2bc6ca33c7e31742d5/developers/proposal_format.md) gets a section below "Status", that has links to the PRs / commits that make the proposal fully "implemented".