docs: add config to docs README

Author: eddyerburgh

docs/en/README.md

@@ -71,3 +71,4 @@
     * [TransitionGroupStub](api/components/TransitionGroupStub.md)
   * [Selectors](api/selectors.md)
   * [createLocalVue](api/createLocalVue.md)
+  * [config](api/config.md)

docs/en/api/config.md

@@ -0,0 +1,25 @@
+# Config
+
+vue-test-utils includes a config object to defined options used by vue-test-utils.
+
+## `vue-test-utils` Config Options
+
+### `stubs`
+
+- type: `Object`
+- default: `{
+  transition: TransitionStub,
+  'transition-group': TransitionGroupStub
+}`
+
+Stubs to use in components. These are overwritten by `stubs` passed in the mounting options.
+
+When passing `stubs` as an array in the mounting options, `config.stubs` are converted to an array, and will stub components with a basic component that returns a div.
+
+Example:
+
+```js
+import VueTestUtils from 'vue-test-utils'
+
+VueTestUtils.config.stubs['my-compomnent'] = '<div />'
+```