Merge pull request #381 from suzil/cleanup/add-badges

Add badges

README.md

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 # Frontera
 
+[![pypi](https://img.shields.io/pypi/v/frontera)](https://pypi.org/project/frontera/)
+[![python versions](https://img.shields.io/pypi/pyversions/frontera.svg)](https://pypi.org/project/frontera/)
+[![Build Status](https://travis-ci.org/scrapinghub/frontera.svg?branch=master)](https://travis-ci.org/scrapinghub/frontera)
+[![codecov](https://codecov.io/gh/scrapinghub/frontera/branch/master/graph/badge.svg)](https://codecov.io/gh/scrapinghub/frontera)
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 ## Overview
 
 Frontera is a web crawling framework consisting of [crawl frontier](http://nlp.stanford.edu/IR-book/html/htmledition/the-url-frontier-1.html), and distribution/scaling primitives, allowing to build a large scale online web crawler.