doc(readme): Add offline

Author: weihanchen

readme.md

@@ -3,6 +3,7 @@ binary-search-tree-js-visualize
 [![Build Status](https://travis-ci.org/weihanchen/binary-search-tree-js-visualize.svg?branch=master)](https://travis-ci.org/weihanchen/binary-search-tree-js-visualize)
 
 Data structure of binary search tree implement with html5 canvas.
+Support offline with pwa cache.
 
 ## Build Requirements
 
@@ -16,6 +17,7 @@ Nodejs ^6.5.0
 * [Webpack](https://webpack.github.io/)
 * [Material Design for Bootstrap](http://fezvrasta.github.io/bootstrap-material-design/)
 * [Sass](http://sass-lang.com/)
+* [Precache](https://github.com/goldhand/sw-precache-webpack-plugin)
 * [CI with travis and github's gh-pages](https://gist.github.com/domenic/ec8b0fc8ab45f39403dd)
 
 ## Run example with server