fix readme

Author: ele828

README.md

@@ -6,6 +6,8 @@ Super thanks to [**promises-aplus-tests**](https://github.com/promises-aplus/pro
 # Usage
 Just use it like any other Promise libraries.
 
+```npm install hi-promise --save```
+
 ```javascript
 var Promise = require('hi-promise');
 var p = new Promise(function(resolve, reject) {});