add one screenshot to test the img feature

docs/honk.1

@@ -64,6 +64,8 @@ It will contain posts from all followed honkers.
 Posts mentioning the user, both followed and not, are collected under the
 .Pa @me
 tab.
+.Pp
+.Lk honk-screenshot.png screen of one honk
 .Ss Honking
 On the home page and most other pages, there will be a button marked
 .Dq it's honking time