add a simple example of sub “prompt”

Author: tbrowder

doc/Language/system.pod6

@@ -42,6 +42,14 @@ C<.lines> will read in turn one line from every one of them, changing the value
 of C<$*ARGFILES.path> every time a new handle is being processed. In general, it
 provides a very convenient API for scripts that deal with sets of files.
 
+=head1 Getting arguments interactively
+
+Use C<prompt> to have a running program query the user for data:
+
+=begin code
+my Uint $num-iters = prompt "How many iterations to run: ";
+=end code
+
 =head1 Running programs synchronously and asynchronously
 
 There are two routines to run external programs: L<C<run>|/routine/run> and