Expand plugin use examples in doc #3

Author: RadValentin

README.md

@@ -11,35 +11,56 @@
 
 Prefix every rule with a selector.
 
-```js
-var css = '.a {} \n.b{}'
+## Installation
+
+```console
+$ npm install postcss-prefix-selector
+```
 
+## Usage
+
+```js
 var prefix = require('postcss-prefix-selector')
 
+// css to be processed
+var css = fs.readFileSync("input.css", "utf8")
+
 var out = postcss().use(prefix({
-  prefix: '.some-selector ' // <--- notice the traililng space!
+  prefix: '.some-selector ', // <--- notice the traililng space!
+  exclude: ['.c']
 })).process(css).css
-
-console.log(out)
 ```
 
-## Options
+Using this `input.css`:
 
-It's possible to avoid prefixing some selectors with the option `exclude` which take an array of selectors in parameter.
+```css
+.a, .b {
+  color: aqua;
+}
 
-```js
-var css = '.a {} \nhtml{} \n.b{}'
+.c {
+  color: coral;
+}
+```
 
-var prefix = require('postcss-prefix-selector')
+you will get:
 
-var out = postcss().use(prefix({
-  prefix: '.some-selector ', // <--- notice the traililng space!
-  exclude: ['html', '.b']
-})).process(css).css
+```css
+.some-selector .a, .some-selector .b {
+  color: aqua;
+}
 
-console.log(out)
+.c {
+  color: coral;
+}
 ```
 
+
+## Options
+
+It's possible to avoid prefixing some selectors by using the `exclude` option which takes an array of selectors as a parameter.
+
+
 [gitter-image]: https://badges.gitter.im/jonathanong/postcss-prefix-selector.png
 [gitter-url]: https://gitter.im/jonathanong/postcss-prefix-selector
 [npm-image]: https://img.shields.io/npm/v/postcss-prefix-selector.svg?style=flat-square