Mention shell and run in the qqx section

Author: paultcochrane

lib/Language/quoting.pod

@@ -207,6 +207,9 @@ Again, the output of the external command can be kept in a variable:
     # runs the command: grep -i cool /usr/share/dict/words
     say $output;      # Cooley␤Cooley's␤Coolidge␤Coolidge's␤cool␤ ...
 
+See also L<shell|/routine/shell> and L<run|/routine/run> for other ways to
+execute external commands.
+
 =head2 Heredocs: :to
 
 TODO