Adding supported version badges (#256)

README.md

 
 [![license badge](https://img.shields.io/badge/license-Apache--2.0-green.svg)](LICENSE.md)
 
+<img src="https://img.shields.io/badge/unity-2019.4-green.svg?style=flat-square" alt="unity 2019.4">
+<img src="https://img.shields.io/badge/unity-2020.2-green.svg?style=flat-square" alt="unity 2020.2">
+
 > com.unity.perception is in active development. Its features and API are subject to significant change as development progresses.