chore: add slack to readme (#5281)

Signed-off-by: Will Manning <[email protected]>

Author: lwwmanning

README.md

@@ -9,7 +9,7 @@
 [![Maven - Version](https://img.shields.io/maven-central/v/dev.vortex/vortex-spark)](https://central.sonatype.com/artifact/dev.vortex/vortex-spark)
 [![codecov](https://codecov.io/github/vortex-data/vortex/graph/badge.svg)](https://codecov.io/github/vortex-data/vortex)
 
-📚 [Documentation](https://docs.vortex.dev/) | 📊 [Performance Benchmarks](https://bench.vortex.dev)
+🫶 [Join the community on Slack!](https://vortex.dev/slack) | 📚 [Documentation](https://docs.vortex.dev/) | 📊 [Performance Benchmarks](https://bench.vortex.dev)
 
 ## Overview