Add polyfill callout to upgrade guide

docs/source/2.0-migration.md

@@ -59,6 +59,12 @@ import gql from 'graphql-tag';
 import { ApolloProvider, graphql } from 'react-apollo';
 ```
 
+<h3 id="fetch">Fetch polyfill</h3>
+
+Apollo Client 2.0 no longer always includes a polyfill for the browser `fetch` function. Read about how to set that up in the [Apollo Link docs](https://www.apollographql.com/docs/link/links/http.html#fetch).
+
+You should include this polyfill to support browsers such as Internet Explorer and Opera Mini. [See the compatbility details here.](https://caniuse.com/#search=fetch)
+
 <h3 id="basic" title="Basic updates">Basic updates</h3>
 We know how much hard work and care you have put into your application! The 2.0 upgrade is designed to be as minimal as possible and to not require any application (like UI integration) changes. It is instead focused on how to construct a client as a way to new features and improvements. A simple usage of Apollo Client upgrading to the 2.0 would look like this: