Use PR description as merge commit body instead of custom message (#3)

Fetch the rerun PR body (which mirrors the universe PR description) and
use it as the squash merge commit body, appending only the Source-Ref
trailer.

Source-Ref: a95316d96e59fa162cd4315fa5ea96c0873bc1f5

Author: jleibs

CONTRIBUTING.md

@@ -1,10 +1,6 @@
 # Contributing to Rerun
 This is written for anyone who wants to contribute to the Rerun repository.
 
-FYI, this is going to do awesome stuff through the universe repo.
-This time with proper attribution.
-
-
 ## See also
 * [`ARCHITECTURE.md`](ARCHITECTURE.md)
 * [`BUILD.md`](BUILD.md)