Add licence badge

Author: ben-marshall

README.md

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 [![Build Status](https://travis-ci.org/ben-marshall/verilog-parser.svg?branch=master)](https://travis-ci.org/ben-marshall/verilog-parser/branches)
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+![Licence: MIT](https://img.shields.io/badge/License-MIT-blue.svg)
 
 This repository contains a flex / bison parser for the IEEE 1364-2001 Verilog
 Standard.