Add note feature to triagebot's config (#14620)

This will let people [add summary
notes](https://forge.rust-lang.org/triagebot/note.html) to an issue, for
example to add that an issue is best kept for first-timers.

changelog: none

I wanted to use it in
#14617 (comment).

Author: flip1995

triagebot.toml

@@ -11,6 +11,8 @@ allow-unauthenticated = [
 
 [merge-conflicts]
 
+[note]
+
 [canonicalize-issue-links]
 
 # Prevents mentions in commits to avoid users being spammed