Add discord shield badge to README (#1332)

This would highlight how active the community really is at just a glance, which is nice. It will however require an admin of the discord server to activate this under Server Settings->Widget-> enable server widget

README.md

@@ -5,6 +5,7 @@
 [![Crates.io](https://img.shields.io/crates/d/bevy.svg)](https://crates.io/crates/bevy)
 [![Rust](https://github.com/bevyengine/bevy/workflows/CI/badge.svg)](https://github.com/bevyengine/bevy/actions)
 ![iOS cron CI](https://github.com/bevyengine/bevy/workflows/iOS%20cron%20CI/badge.svg)
+[![Discord](https://img.shields.io/discord/691052431525675048.svg?label=&logo=discord&logoColor=ffffff&color=7389D8&labelColor=6A7EC2)](https://discord.gg/gMUk5Ph)
 
 ## What is Bevy?