README.md: Add gem badge (#7)

Author: PetrHeinz

README.md

@@ -5,6 +5,7 @@
 [![Better Stack dashboard](https://github.com/logtail/logtail-python/assets/10132717/e2a1196b-7924-4abc-9b85-055e17b5d499)](https://betterstack.com/logs)
 
 [![ISC License](https://img.shields.io/badge/license-ISC-ff69b4.svg)](LICENSE.md)
+[![Gem Version](https://badge.fury.io/rb/fluent-plugin-logtail.svg)](https://badge.fury.io/rb/fluent-plugin-logtail)
 [![build](https://github.com/logtail/fluentd-plugin-logtail/actions/workflows/main.yml/badge.svg?branch=main)](https://github.com/logtail/fluentd-plugin-logtail/actions/workflows/main.yml)
 
 Experience SQL-compatible structured log management based on ClickHouse. [Learn more ⇗](https://betterstack.com/logs)