Add Spectrum badge to readme (#2286)

README.md

@@ -7,6 +7,7 @@ _Simple, scalable state management_
 [![CircleCI](https://circleci.com/gh/mobxjs/mobx.svg?style=svg)](https://circleci.com/gh/mobxjs/mobx)
 [![Coverage Status](https://coveralls.io/repos/mobxjs/mobx/badge.svg?branch=master&service=github)](https://coveralls.io/github/mobxjs/mobx?branch=master)
 [![Join the chat at https://gitter.im/mobxjs/mobx](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/mobxjs/mobx?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Join the community on Spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/mobx)
 [![Discuss MobX on Hashnode](https://hashnode.github.io/badges/mobx.svg)](https://hashnode.com/n/mobx)
 [![OpenCollective](https://opencollective.com/mobx/backers/badge.svg)](#backers)
 [![OpenCollective](https://opencollective.com/mobx/sponsors/badge.svg)](#sponsors)