triagebot: autolabel merged PRs with release-notes

`release-notes` is intended to be manually removed if a merged PR does
not need relnotes.

Author: jieyouxu

triagebot.toml

@@ -22,6 +22,11 @@ exclude_labels = [
     "C-tracking-issue",
 ]
 
+# By default, we will tag merged PRs with `release-notes`; they can be removed
+# manually if we determine that a PR does not need relnotes.
+[autolabel."release-notes"]
+pr_merged = true
+
 [autolabel."A-CI"]
 trigger_files = [
     ".github/workflows",