chore(readme): add GoDoc badge

README.md

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 [![Build Status](https://travis-ci.org/cad/ovpm.svg?branch=master)](https://travis-ci.org/cad/ovpm)
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 OVPM allows you to manage an OpenVPN server from command line easily. With OVPM you can create and run an OpenVPN server, add/remove vpn users, generate client .ovpn files for your users etc.