doc: Update API version badge (#339)

References:

  * PR: https://github.com/yagop/node-telegram-bot-api/pull/339
  * PR-by: @kamikazechaser 
  * API v3 progress tracker: https://github.com/yagop/node-telegram-bot-api/issues/332

README.md

@@ -1,6 +1,6 @@
 # Node.js Telegram Bot API
 
-[![Bot API](http://img.shields.io/badge/Bot%20API-v2.3.1-00aced.svg)](https://core.telegram.org/bots/api)
+[![Bot API](http://img.shields.io/badge/Bot%20API-v3.0.0-00aced.svg)](https://core.telegram.org/bots/api)
 [![Build Status](https://travis-ci.org/yagop/node-telegram-bot-api.svg?branch=master)](https://travis-ci.org/yagop/node-telegram-bot-api)
 [![Build status](https://ci.appveyor.com/api/projects/status/ujko6bsum3g5msjh/branch/master?svg=true)](https://ci.appveyor.com/project/yagop/node-telegram-bot-api/branch/master)
 [![Coverage Status](https://coveralls.io/repos/yagop/node-telegram-bot-api/badge.svg?branch=master)](https://coveralls.io/r/yagop/node-telegram-bot-api?branch=master)