update README

README.md

@@ -95,12 +95,17 @@ rollup.rollup({
   // The bundle's starting point
   entry: 'app.js',
 
+  // Any external modules you don't want to include
+  // in the bundle (includes node built-ins)
+  external: [ 'path', 'fs', 'some-other-lib' ],
+
   // Override the default path resolution
-  resolvePath: function ( importee, importer ) {
+  resolvePath: function ( importee, importer, options ) {
     // return a string or a falsy value - if falsy,
-    // import is kept external to the bundle.
-    // Alternative, return a Promise that fulfils
-    // with a string or falsy value
+    // import is kept external to the bundle. Alternatively,
+    // return a Promise that fulfils with a string or falsy
+    // value. `options` is whatever you passed to `rollup`
+    // initially
   }
 }).then( function ( bundle ) {
   // generate code and a sourcemap