add clarifying comment

lib/bundler.js

@@ -419,6 +419,7 @@ _.extend(Bundle.prototype, {
   // directory within the bundle
   bundleNodeModules: function (pkg) {
     var nodeModulesPath = path.join(pkg.npmDir(), 'node_modules');
+    // use '/' rather than path.join since this is part of a url
     var relNodeModulesPath = ['packages', pkg.name, 'node_modules'].join('/');
     this.nodeModulesDirs[relNodeModulesPath] = nodeModulesPath;
   },