Added Gitter badge

Author: gitter-badger

Readme.md

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 Set up a PHP development box super fast
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+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/dirkaholic/vagrant-php-dev-box?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 Installation
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