message: Document the relnotes label too
diff: @@ -69,6 +69,9 @@ labels to triage issues:
  are only assigned during triage meetings, and replace the [I-nominated][inom]
  label.
  +* The light orange **relnotes** label marks issues that should be documented in
  + the release notes of the next release.
  +
  If you're looking for somewhere to start, check out the [E-easy][eeasy] tag.
  [eeasy]: https://github.com/rust-lang/cargo/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy
  