Added frontend GraphQL docs

doc/development/fe_guide/graphql.md

+# GraphQL
+
+We use [Apollo] and [Vue Apollo][vue-apollo] for working with GraphQL
+on the frontend.
+
+In order to use GraphQL, you need to enable the `graphql` feature flag,
+read more about [Feature Flags][feature-flags].
+
+## Apollo Client
+
+To save duplicated clients getting created in different apps, we have a
+[default client][defualt-client] that should be used. This setups the
+Apollo client with the correct URL and also sets the CSRF headers.
+
+## GraphQL Queries
+
+To save query compilation at runtime, webpack can directly import `.graphql`
+files. This allows webpack to preprocess the query at compile time instead
+of the client doing compilation of queries.
+
+## Usage in Vue
+
+To use Vue Apollo, import the [Vue Apollo][vue-apollo] plugin as well
+as the default client. This should be created at the same point
+the Vue application is mounted.
+
+```javascript
+import Vue from 'vue';
+import VueApollo from 'vue-apollo';
+import defaultClient from '~/lib/graphql';
+Vue.use(VueApollo);
+
+const apolloProvider = new VueApollo({
+  defaultClient,
+});
+
+new Vue({
+  ...,
+  apolloProvider,
+  ...
+});
+```
+
+Read more about [Vue Apollo][vue-apollo] in the [Vue Apollo documentation][vue-apollo-docs].
+
+### Testing
+
+With [Vue test utils][vue-test-utils] it is easy to quickly test components that
+fetch GraphQL queries. The simplest way is to use `shallowMount` and then set
+the data on the component
+
+```javascript
+it('tests apollo component', () => {
+  const vm = shallowMount(App);
+
+  vm.setData({
+    ...mock data
+  });
+});
+```
+
+## Usage outside of Vue
+
+It is also possible to use GraphQL outside of Vue by directly importing
+and using the default client with queries.
+
+```javascript
+import defaultClient from '~/lib/graphql';
+import query from './query.graphql';
+
+defaultClient.query(query)
+  .then(result => console.log(result));
+```
+
+Read more about the [Apollo] client in the [Apollo documentation][apollo-client-docs].
+
+[Apollo]: https://www.apollographql.com/
+[vue-apollo]: https://github.com/Akryum/vue-apollo/
+[vue-apollo-docs]: https://akryum.github.io/vue-apollo/
+[feature-flags]: ../feature_flags.md
+[default-client]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/lib/graphql.js
+[apollo-client-docs]: https://www.apollographql.com/docs/tutorial/client.html
+[vue-test-utils]: https://vue-test-utils.vuejs.org/

doc/development/fe_guide/index.md

@@ -54,6 +54,9 @@ Vuex specific design patterns and practices.
 ## [Axios](axios.md)
 Axios specific practices and gotchas.
 
+## [GraphQL](graphql.md)
+How to use GraphQL
+
 ## [Icons and Illustrations](icons.md)
 How we use SVG for our Icons and Illustrations.