add comments support (#7)

rpn.pod

@@ -213,6 +213,16 @@ Search through history.
 
 =back
 
+=head1 COMMENTS
+
+Lines starting with  C<#> are being ignored as comments.  You can also
+append comments to rpn input, e.g.:
+
+   # a comment
+   123   # another comment
+
+In this case only 123 will be added to the stack.
+
 =head1 EXTENDING RPN USING LUA
 
 You can use a lua script with lua functions to extend the