Update readme with usage notes

Author: bartveneman

README.md

@@ -1,2 +1,37 @@
-# css-analyzer-diff
-Calculate the difference between two sets of CSS stats
+# css-analyzer-diff [![Build Status](https://travis-ci.org/bartveneman/css-analyzer-diff.svg?branch=master)](https://travis-ci.org/bartveneman/css-analyzer-diff) [![Known Vulnerabilities](https://snyk.io/test/github/bartveneman/css-analyzer-diff/badge.svg)](https://snyk.io/test/github/bartveneman/css-analyzer-diff) ![Dependencies Status](https://img.shields.io/david/bartveneman/css-analyzer-diff.svg) ![Dependencies Status](https://img.shields.io/david/dev/bartveneman/css-analyzer-diff.svg)
+
+Calculate the difference between two sets of [CSS stats](https://github.com/projectwallace/css-analyzer).
+
+## Usage
+
+```js
+const differ = require('css-analyzer-diff');
+const analyzeCss = require('@projectwallace/css-analyzer');
+
+const [firstStats, secondStats] = await Promise.all([
+  analyzeCss('.cat { color: brown; }'),
+  analyzeCss('.cat { color: red; }')]
+);
+const changes = differ(firstStats, secondStats);
+
+//=> Returns a Map with all changes
+// Map {
+//   'values.colors.unique' => [
+//     {
+//       value: 'brown',
+//       removed: true,
+//       added: false,
+//       changed: true
+//     },
+//     {
+//       value: 'red',
+//       removed: false,
+//       added: true,
+//       changed: true
+//     }
+//  ],
+//
+//  ... many more ...
+//
+// }
+```