;tags: mention pivot trick for listing tag values

[ci skip]

hledger/Hledger/Cli/Commands/Tags.md

@@ -4,3 +4,8 @@ only tag names matching the regular expression (case insensitive) are shown.
 With QUERY arguments, only transactions matching the query are considered.  
 
 _FLAGS_
+
+There's no direct way to list a tag's values, but there is an indirect way:
+--pivot converts a tag's values to accounts, which you can list, like this:
+
+    hledger --pivot SOMETAG accounts --used