message: chore(readme): Update Slack badge
diff: [![Test Coverage](https://api.codeclimate.com/v1/badges/cb5ec42a2d0cc1a47a02/test_coverage)](https://codeclimate.com/github/feathersjs/feathers/test_coverage)
  [![Download Status](https://img.shields.io/npm/dm/@feathersjs/feathers.svg?style=flat-square)](https://www.npmjs.com/package/@feathersjs/feathers)
  -[![Slack Status](http://slack.feathersjs.com/badge.svg)](http://slack.feathersjs.com)
  +[![Slack Status](https://img.shields.io/badge/Slack-Join-brightgreen)](http://slack.feathersjs.com)
  [![Telegram Status](https://img.shields.io/badge/Telegram_RU_chat:-Feathers-216bc1.svg?style=flat)](https://t.me/featherjs)
  Feathers is a lightweight web-framework for creating real-time applications and REST APIs using JavaScript or TypeScript.
  