docs: update readme with simple explanation

Author: patrickhulce

README.md

@@ -5,4 +5,41 @@
 [![Commitizen friendly](https://img.shields.io/badge/commitizen-friendly-brightgreen.svg)](http://commitizen.github.io/cz-cli/)
 [![Dependencies](https://david-dm.org/patrickhulce/nukecss.svg)](https://david-dm.org/patrickhulce/nukecss)
 
-Eliminates unused CSS rules.
+Eliminates unused CSS rules. Built for single-page apps from the ground up. Inspired by [purifycss](https://github.com/purifycss/purifycss) and [uncss](https://github.com/giakki/uncss).
+
+## How It Works
+* Parses the CSS with [gonzales-pe](https://github.com/tonyganch/gonzales-pe) and walks the AST to find the IDs, classes, and DOM types used in selectors.
+* Parses HTML and JavaScript sources to find rule usage in strings and attributes, falling back to simple RegExp search when parsing fails.
+* Removes rules whose selectors cannot be found in the source set.
+
+## Usage
+`npm install --save nukecss`
+
+```js
+const nukecss = require('nukecss')
+
+const javascript = 'const jsignored = "js-class other-class"'
+const javascript2 = 'const woah = ["still", "works"].join("-")'
+const html = '<div id="primary" class="html-class">html-ignored</div>'
+const css = `
+.jsignored { color: white; }
+.html-ignored { color: white; }
+.js-class { color: white; }
+.other-class { color: white; }
+.still-works { color: white; }
+#primary { color: white; }
+#primary > .unused { color: white; }
+.also-unused { color: white; }
+`
+
+const nuked = nukecss([
+  {content: javascript, type: 'js'},
+  {content: javascript2, type: 'js'},
+  {content: html, type: 'html'},
+], css)
+> console.log(nuked)
+.js-class { color: white; }
+.other-class { color: white; }
+.still-works { color: white; }
+#primary { color: white; }
+```