Add async guide

docs/en/guides/testing-async-components.md

@@ -0,0 +1,81 @@
+# Testing Asynchronous Behavior
+
+To simplify testing, `vue-test-utils` applies updates _synchronously_. However, there are some techniques you need to be aware of when testing a component with asynchronous behavior such as callbacks or promises.
+
+One of the most common asynchronous behaviors is API calls and Vuex actions. The following examples shows how to test a method that makes an API call. This example is using Jest to run the test and to mock the HTTP library `axios`.
+
+The implementation of the `axios` mock looks like this:
+
+``` js
+export default {
+  get: () => new Promise(resolve => {
+    resolve({ data: 'value' })
+  })
+}
+```
+
+The below component makes an API call when a button is clicked, then assigns the response to `value`.
+
+``` js
+<template>
+  <button @click="fetchResults" />
+</template>
+
+<script>
+  import axios from 'axios'
+  export default {
+    data () {
+      return {
+        value: null
+      }
+    },
+
+    methods: {
+      async fetchResults () {
+        const response = await axios.get('mock/service')
+        this.value = response.data
+      }
+    }
+  }
+</script>
+```
+A test can be written like this:
+
+``` js
+import { shallow } from 'vue-test-utils'
+import Foo from './Foo'
+jest.mock('axios')
+
+test('Foo', () => {
+  it('fetches async when a button is clicked', () => {
+    const wrapper = shallow(Foo)
+    wrapper.find('button').trigger('click')
+    expect(wrapper.vm.value).toEqual('value')
+  })
+})
+```
+
+This test currently fails, because the assertion is called before the promise resolves. One solution is to use the npm package, `flush-promises`. which immediately resolve any unresolved promises. This test is also asynchronous, so like the previous example, we need to let the test runner know to wait before making any assertions. 
+
+If you are using Jest, there are a few options, such as passing a `done` callback, as shown above. Another is to prepend the test with the ES7 'async' keyword. We can now use the the ES7 `await` keyword with `flushPromises`, to immediately resolve the API call.
+
+The updated test looks like this:
+
+``` js
+import { shallow } from 'vue-test-utils'
+import flushPromises from 'flush-promises'
+import Foo from './Foo'
+jest.mock('axios')
+
+test('Foo', () => {
+  it('fetches async when a button is clicked', () => {
+    const wrapper = shallow(Foo)
+    wrapper.find('button').trigger('click')
+    await flushPromises()
+    expect(wrapper.vm.value).toEqual('value')
+  })
+})
+```
+
+This same technique can be applied to Vuex actions, which return a promise by default.
+