Add Discord invite badge to README (#1046)

Meant to do this a while ago...

README.md

@@ -4,6 +4,7 @@
 [![Documentation](https://img.shields.io/badge/view-docs-blue)](https://theseus-os.github.io/Theseus/doc/___Theseus_Crates___/index.html)
 [![Book](https://img.shields.io/badge/view-book-blueviolet)](https://theseus-os.github.io/Theseus/book/index.html)
 [![Blog](https://img.shields.io/badge/view-blog-orange)](https://theseus-os.com)
+[![Discord](https://img.shields.io/badge/Discord-%235865F2.svg?style=flat&logo=discord&logoColor=white)](https://discord.gg/NuUnqeYT8R)
 
 Theseus is a new OS written from scratch in [Rust](https://www.rust-lang.org/) to experiment with novel OS structure, better state management, and how to leverage **intralingual design** principles to shift OS responsibilities like resource management into the compiler.