add XO and NPM version badges. rename readme to lowercase (#15)

Author: bartveneman

README.md

@@ -0,0 +1,39 @@
+# css-analyzer-diff
+
+[![NPM Version](https://img.shields.io/npm/v/@projectwallace/css-analyzer.svg)](https://www.npmjs.com/package/@projectwallace/css-analyzer) [![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) [![XO code style](https://img.shields.io/badge/code_style-XO-5ed9c7.svg)](https://github.com/sindresorhus/xo)
+
+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 an object with all changes
+// {
+//   'values.colors.unique': [
+//     {
+//       value: 'brown',
+//       removed: true,
+//       added: false,
+//       changed: true
+//     },
+//     {
+//       value: 'red',
+//       removed: false,
+//       added: true,
+//       changed: true
+//     }
+//  ],
+//
+//  ... many more ...
+//
+// }
+```