docs: mention that vuex requires promise (#1033)

* docs: mention that vuex requires promise

* docs: add comma before "such as"

Author: ktsn

docs/en/installation.md

@@ -38,6 +38,31 @@ Vue.use(Vuex)
 
 You don't need to do this when using global script tags.
 
+### Promise
+
+Vuex requires [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises). If your supporting browsers do not implement Promise (e.g. IE), you can use a polyfill library, such as [es6-promise](https://github.com/stefanpenner/es6-promise).
+
+You can include it via CDN:
+
+``` html
+<script src="https://cdn.jsdelivr.net/npm/es6-promise@4/dist/es6-promise.auto.js"></script>
+```
+
+Then `window.Promise` will be available automatically.
+
+If you prefer using a package manager such as NPM or Yarn, install it with the following commands:
+
+``` bash
+npm install es6-promise --save # NPM
+yarn add es6-promise # Yarn
+```
+
+Furthermore, add the below line into anywhere in your code before using Vuex:
+
+``` js
+import 'es6-promise/auto'
+```
+
 ### Dev Build
 
 You will have to clone directly from GitHub and build `vuex` yourself if