readme: add telegram discuss group

README.md

@@ -4,6 +4,7 @@
 [![GoDoc](https://godoc.org/gopkg.in/tucnak/telebot.v2?status.svg)](https://godoc.org/gopkg.in/tucnak/telebot.v2)
 [![Travis](https://travis-ci.org/tucnak/telebot.svg?branch=v2)](https://travis-ci.org/tucnak/telebot)
 [![codecov.io](https://codecov.io/gh/tucnak/telebot/coverage.svg?branch=develop)](https://codecov.io/gh/tucnak/telebot)
+[![Discuss on Telegram](https://img.shields.io/badge/telegram-discuss-0088cc.svg)](https://t.me/go_telebot)
 
 ```bash
 go get -u gopkg.in/tucnak/telebot.v2