Add example of feeding Proc's STDIN from a string

Author: zoffixznet

doc/Type/Proc.pod6

@@ -22,6 +22,16 @@ second command, you can do
     my $p2 = run 'cat', '-n', :in($p1.out), :out;
     say $p2.out.get;
 
+You can also feed the `:in` pipe directly from your program, by setting it
+to `True`, which will make the pipe available via `.in` method on the C<Proc>:
+
+    my $p = run "cat", "-n", :in, :out;
+    $p.in.say: "Hello,\nworld!";
+    $p.in.close;
+    say $p.out.slurp-rest;
+    # OUTPUT:
+    #    1  Hello,
+    #    2  world!
 
 =head1 Methods