Add Twitter badge on main readme (#282)

Signed-off-by: Tihomir Surdilovic <[email protected]>

Author: Tihomir Surdilovic

README.md

@@ -2,6 +2,7 @@
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/serverlessworkflow/specification/blob/master/LICENSE)
 [<img src="http://img.shields.io/badge/slack-@serverless--workflow-red?style=social&logo=slack">](https://cloud-native.slack.com/messages/serverless-workflow) 
 [<img src="http://img.shields.io/badge/web-serverlessworkflow.io-red?style=social&logo=google-chrome">](https://cloud-native.slack.com/messages/serverless-workflow) 
+[<img src="https://img.shields.io/twitter/follow/CNCFWorkflow?style=social">](https://twitter.com/CNCFWorkflow)
 
 <p align="center">
 <img src="media/logos/serverlessworkflow-horizontal-color.png" width="500px" alt="Serverless Workflow Specification"/>