document depCache idea in builder API

docs/api.md

@@ -109,14 +109,15 @@ builder.config({ custom: 'options' });
 
 // or builder.buildStatic
 builder.bundle('app/main.js', {
-  minify: true,
-
-  // inject the bundle config into the configuration file
-  inject: true
+  minify: true
 })
 .then(function(output) {
   // output is now an in-memory build
-})
+  // output.source
+
+  // get the depCache configuration for the tree
+  var depCache = builder.getDepCache(output.tree);
+});
 ```
 
 The builder will be automatically configured to have the correct jspm configuration and baseURL for the environment.