doc improvement

src/test/java/org/kohsuke/HookApp.java

@@ -9,6 +9,9 @@ import java.io.IOException;
 import java.io.StringReader;
 
 /**
+ * App to test the hook script. You need some internet-facing server that can forward the request to you
+ * (typically via SSH reverse port forwarding.)
+ *
  * @author Kohsuke Kawaguchi
  */
 public class HookApp {