add latest version to README (#4960)

README.md

 ## PX4 Pro Drone Autopilot ##
 
-[![DOI](https://zenodo.org/badge/22634/PX4/Firmware.svg)](https://zenodo.org/badge/latestdoi/22634/PX4/Firmware) [![Build Status](https://travis-ci.org/PX4/Firmware.svg?branch=master)](https://travis-ci.org/PX4/Firmware) [![Coverity Scan](https://scan.coverity.com/projects/3966/badge.svg?flat=1)](https://scan.coverity.com/projects/3966?tab=overview)
+[![Releases](https://img.shields.io/github/release/PX4/Firmware.svg)](https://github.com/PX4/Firmware/releases) [![DOI](https://zenodo.org/badge/22634/PX4/Firmware.svg)](https://zenodo.org/badge/latestdoi/22634/PX4/Firmware) [![Build Status](https://travis-ci.org/PX4/Firmware.svg?branch=master)](https://travis-ci.org/PX4/Firmware) [![Coverity Scan](https://scan.coverity.com/projects/3966/badge.svg?flat=1)](https://scan.coverity.com/projects/3966?tab=overview)
 
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