Add Pypi badge to README

Author: illusional

README.md

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 # python-wdlgen
 
 [![Build Status](https://travis-ci.org/illusional/python-wdlgen.svg?branch=master)](https://travis-ci.org/illusional/python-wdlgen)
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+[![PyPI version](https://badge.fury.io/py/illusional.wdlgen.svg)](https://badge.fury.io/py/illusional.wdlgen)
+ 
 [Workflow Description Language](http://www.openwdl.org) is way to describe tasks and workflows in a "_human readable and writable way_". It was initially developed and offered by [Broad Institute](https://software.broadinstitute.org/) to be paired with their workflow engine [Cromwell](https://cromwell.readthedocs.io/en/stable/), however it has since been made open source with other engines such as [Toil](http://toil.readthedocs.io/en/3.14.0/running/wdl.html) and [DNAnexus\*](https://github.com/dnanexus/dxWDL).