Added Discuss badge to README (#6695)

README.md

-Nomad [![Build Status](https://circleci.com/gh/hashicorp/nomad.svg?style=svg)](https://circleci.com/gh/hashicorp/nomad) [![Join the chat at https://gitter.im/hashicorp-nomad/Lobby](https://badges.gitter.im/hashicorp-nomad/Lobby.svg)](https://gitter.im/hashicorp-nomad/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+Nomad [![Build Status](https://circleci.com/gh/hashicorp/nomad.svg?style=svg)](https://circleci.com/gh/hashicorp/nomad) [![Discuss](https://img.shields.io/badge/discuss-nomad-00BC7F?style=flat)](https://discuss.hashicorp.com/c/nomad)
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