Add Object.assign reference to _.assign docs. [ci skip]

lodash.src.js

@@ -9205,7 +9205,9 @@
      * The `customizer` is bound to `thisArg` and invoked with five arguments:
      * (objectValue, sourceValue, key, object, source).
      *
-     * **Note:** This method mutates `object`.
+     * **Note:** This method mutates `object` and is based on
+     * [`Object.assign`](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.assign).
+     *
      *
      * @static
      * @memberOf _