docs: added extra example for url.filter for a common scenario (webpack#1428)

Author: dovidweisz

README.md

@@ -161,6 +161,11 @@ module.exports = {
                 return false;
               }
 
+              // Don't handle images under root-relatve /external_images/
+              if (/^\/external_images\//.test(path)) {
+                return false;
+              }
+
               return true;
             },
           },