Add a Gitter chat badge to README.md

### Habemus sala de chat 

Nodeschool/montevideo now has a Chat Room on Gitter

We have created a chat room. You can visit it here: [https://gitter.im/nodeschool/montevideo](https://gitter.im/nodeschool/montevideo).

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/nodeschool/montevideo?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=body_badge)

Happy chatting.