doc: add a badge / link to the latest stable release

README.md

 
 ## Download
 
+| Latest stable release | [![release](https://img.shields.io/badge/release-latest-green.svg)](https://github.com/jitsi/jitsi-meet/releases/latest) |
+|---|---|
+
 You can download Debian/Ubuntu binaries:
 * [stable](https://download.jitsi.org/stable/) ([instructions](https://jitsi.org/downloads/ubuntu-debian-installations-instructions/))
 * [testing](https://download.jitsi.org/testing/) ([instructions](https://jitsi.org/downloads/ubuntu-debian-installations-instructions-for-testing/))