docs: add an example about using 'provide' option. (#928)

Author: xlfsummer

docs/api/options.md

@@ -262,6 +262,25 @@ expect(wrapper.vm.$parent.name).toBe('foo')
 
 Pass properties for components to use in injection. See [provide/inject](https://vuejs.org/v2/api/#provide-inject).
 
+Example:
+
+```js
+const Component = {
+  inject: ['foo'],
+  template: '<div>{{this.foo()}}</div>'
+}
+
+const wrapper = shallowMount(Component, {
+  provide: {
+    foo () {
+      return 'fooValue'
+    }
+  }
+})
+
+expect(wrapper.text()).toBe('fooValue')
+```
+
 ## sync
 
 - type: `boolean`