Add the licensed Joke Shop reference board

CREDITS.md

@@ -58,6 +58,9 @@ their folder sidecar:
 - `docs/screens/refs/hollywood/premiere-staging-street-day-wikimedia-2006-02-12.jpg` — John
   Barreiros, CC BY 2.0, via Wikimedia Commons.
 - Public-domain files in the same folder: Charles W. Beam (1922) and Gary Minnaert (2006).
+- `docs/screens/refs/joke-shop/comedy-store-signature-band-day-wikimedia-2020-04-09.jpg` — Tony
+  Mariotti, CC BY 2.0, via Wikimedia Commons (<https://www.rubyhome.com/> as requested).
+- Public-domain files in the Joke Shop folder: Gary Minnaert (2006).
 
 ## AI-assisted images