Add docs for using Promises and making AJAX requests

template/README.md

@@ -21,6 +21,7 @@ You can find the most recent version of this guide [here](https://github.com/fac
 - [Adding Bootstrap](#adding-bootstrap)
 - [Adding Flow](#adding-flow)
 - [Adding Custom Environment Variables](#adding-custom-environment-variables)
+- [Fetching AJAX Requests](#fetching-ajax-requests)
 - [Integrating with a Node Backend](#integrating-with-a-node-backend)
 - [Proxying API Requests in Development](#proxying-api-requests-in-development)
 - [Adding `<meta>` Tags](#adding-meta-tags)
@@ -463,6 +464,62 @@ if (process.env.NODE_ENV !== 'production') {
 }
 ```
 
+## Fetching AJAX Requests
+
+This project includes a [fetch polyfill](https://github.com/github/fetch), which makes it easier to make web requests.
+
+The `fetch` function allows to easily makes AJAX requests. It takes in a URL as an input and returns a `Promise` that resolves to a `Response` object. You can find more information about `fetch` [here](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch).
+
+
+__About Promises:__ This project also includes a [Promise polyfill](https://github.com/then/promise) which provides a full implementation of Promises/A+. A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises [here](https://www.promisejs.org/) and [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
+
+You can make a GET request like this:
+```javascript
+class MyComponent extends Component {
+  state = {}
+
+  componentDidMount() {
+    this.fetchGists();
+  }
+
+  fetchGists() {
+    fetch('https://api.github.com/users/octocat/gists')
+      .then((response) => response.json())
+      .then((gists) => this.setState({ gists }))
+      .catch((error) => console.log('Error', error));
+  }
+
+  render() {
+    return <UserGists gists={this.state.gists} />;
+  }
+}
+```
+
+You can also use the `async/await` syntax to fetch data. [Here](https://zeit.co/blog/async-and-await) is an introduction to it.
+```javascript
+class MyComponent extends Component {
+  state = {}
+
+  componentDidMount() {
+    this.fetchGists();
+  }
+
+  async fetchGists() {
+    try {
+      const response = await fetch('https://api.github.com/users/octocat/gists');
+      const gists = await response.json();
+      this.setState({ gists });
+    } catch(error) {
+      console.log('Error', error);
+    }
+  }
+
+  render() {
+    return <UserGists gists={this.state.gists} />;
+  }
+}
+```
+
 ## Integrating with a Node Backend
 
 Check out [this tutorial](https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/) for instructions on integrating an app with a Node backend running on another port, and using `fetch()` to access it. You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo).