Ref selection doc updates

issue: vuejs#177

Author: matt-oconnell

docs/en/api/selectors.md

@@ -1,6 +1,6 @@
 # Selectors
 
-A lot of methods take a selector as an argument. A selector can either be a CSS selector, or a Vue component.
+A lot of methods take a selector as an argument. A selector can either be a CSS selector, a Vue component, or a find option object.
 
 ## CSS Selectors
 
@@ -41,3 +41,14 @@ import Foo from './Foo.vue'
 const wrapper = shallow(Foo)
 expect(wrapper.is(Foo)).toBe(true)
 ```
+
+## Find Option Object
+
+### Ref
+
+Using a find option object, vue-test-utils allows for selecting elements by $ref on wrapper components.
+
+```js
+const buttonWrapper = wrapper.find({ ref: 'myButton' });
+buttonWrapper.trigger('click');
+```