Add babel-polyfill to usage example

I ran into this issue too: GitbookIO#1
bfirsh · May 29, 2018 · 29b5f43 · 29b5f43
1 parent 6d8c063
commit 29b5f43
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diff --git a/README.md b/README.md
@@ -13,6 +13,7 @@ $ yarn add expect-firestore
 ## Usage
 
 ```js
+import 'babel-polyfill';
 import assert from 'assert';
 import * as firestore from 'expect-firestore';