Document --external to lib/help.txt

Author: hakatashi

lib/help.txt

@@ -69,6 +69,9 @@ Defaults "entry" to "./test/*.js".
 --no-browser-field   Turn off package.json browser field resolution. This is
                      also handy if you need to run a bundle in node.
 
+        --external   Mark given path or module as external resource and
+                     prevent from being loaded into the bundle.
+
       -y, --yields   Changes the yield interval to allow pending I/O to happen.
 
      -v, --version   Print mochify version and exit.