Docs: Add badges (#326)

Author: Hirse

README.md

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+[![npm version](https://img.shields.io/npm/v/espree.svg)](https://www.npmjs.com/package/espree)
+[![Build Status](https://travis-ci.org/eslint/espree.svg?branch=master)](https://travis-ci.org/eslint/espree)
+[![npm downloads](https://img.shields.io/npm/dm/espree.svg)](https://www.npmjs.com/package/espree)
+[![Bountysource](https://www.bountysource.com/badge/tracker?tracker_id=9348450)](https://www.bountysource.com/trackers/9348450-eslint?utm_source=9348450&utm_medium=shield&utm_campaign=TRACKER_BADGE)
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 # Espree
 
 Espree started out as a fork of [Esprima](http://esprima.org) v1.2.2, the last stable published released of Esprima before work on ECMAScript 6 began. Espree is now built on top of [Acorn](https://github.com/ternjs/acorn), which has a modular architecture that allows extension of core functionality. The goal of Espree is to produce output that is similar to Esprima with a similar API so that it can be used in place of Esprima.