Update README

Author: Ffloriel

packages/rollup-plugin-purgecss/README.md

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+# rollup-plugin-purgecss  
+[![Build Status](https://travis-ci.org/FullHuman/rollup-plugin-purgecss.svg?branch=master)](https://travis-ci.org/FullHuman/rollup-plugin-purgecss) [![CircleCi](https://circleci.com/gh/FullHuman/rollup-plugin-purgecss/tree/master.svg?style=shield)]() [![dependencies Status](https://david-dm.org/fullhuman/rollup-plugin-purgecss/status.svg)](https://david-dm.org/fullhuman/rollup-plugin-purgecss) [![devDependencies Status](https://david-dm.org/fullhuman/rollup-plugin-purgecss/dev-status.svg)](https://david-dm.org/fullhuman/rollup-plugin-purgecss?type=dev)
+
+[Rollup](https://github.com/rollup/rollup) plugin to remove unused css.
+
+## Install
+
+```sh
+npm i rollup-plugin-purgecss -D
+```
+
+## Usage
+
+```js
+import { rollup } from 'rollup';
+import purgecss from 'rollup-plugin-purgecss';
+
+rollup({
+    entry: 'main.js',
+    plugins: [
+        purgecss({
+            content: ["index.html"]
+        })
+    ]
+});
+```
+
+## Contributing
+
+Please read [CONTRIBUTING.md](./.github/CONTRIBUTING.md) for details on our code of conduct, and the process for submitting pull requests to us.
+
+## Versioning
+
+We use [SemVer](http://semver.org/) for versioning. 
+
+## License
+
+This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details