Update README.md

README.md

 # markdown-it-imsize
 
 [![Build Status](https://travis-ci.org/tatsy/markdown-it-imsize.svg?branch=master)](https://travis-ci.org/tatsy/markdown-it-imsize) 
+[![NPM version](https://img.shields.io/npm/v/markdown-it-imsize.svg?style=flat)](https://www.npmjs.org/package/markdown-it-imsize)
 [![Coverage Status](https://coveralls.io/repos/tatsy/markdown-it-imsize/badge.svg)](https://coveralls.io/r/tatsy/markdown-it-imsize)
 
 > markdown-it plugin for size-specified image markups.
+
+## Usage
+
+#### Enable plugin
+
+```js
+var md = require('markdown-it')({
+  html: true,
+  linkify: true,
+  typography: true
+}).use(require('markdown-it-imsize'); // <-- this use(package_name) is required
+```
+
+#### Example
+
+```md
+![test](image.png =100x200)
+```
+
+is interpreted as
+
+```html
+<p><img src="image.png" width="100" height="200"></p>
+```