README.md: add gem version badge

Author: PetrHeinz

README.md

@@ -1,6 +1,7 @@
 # 🪵 Logtail Integration For Rack
 
 [![ISC License](https://img.shields.io/badge/license-ISC-ff69b4.svg)](LICENSE.md)
+[![Gem Version](https://badge.fury.io/rb/logtail-rack.svg)](https://badge.fury.io/rb/logtail-rack)
 [![Build Status](https://github.com/logtail/logtail-ruby-rack/workflows/build/badge.svg)](https://github.com/logtail/logtail-ruby-rack/actions?query=workflow%3Abuild)
 
 This library integrates the [`logtail` Ruby library](https://github.com/logtail/logtail-ruby) with the [rack](https://github.com/rack/rack) framework,