README.md: Add badges

Based on a pull request by Adrien Becchis (@AdrieanKhisbe):

  https://github.com/pegjs/pegjs/pull/392

README.md

@@ -1,3 +1,8 @@
+[![Build status](https://img.shields.io/travis/pegjs/pegjs.svg)](https://travis-ci.org/pegjs/pegjs)
+[![npm version](https://img.shields.io/npm/v/npm.svg)](https://www.npmjs.com/package/pegjs)
+[![Bower version](https://img.shields.io/bower/v/bootstrap.svg)](https://github.com/pegjs/bower)
+[![License](https://img.shields.io/badge/license-mit-blue.svg)](https://opensource.org/licenses/MIT)
+
 PEG.js
 ======