README: centred header with downloads and badges; link the Discord (#1761)

Thanks @bherbruck.

CONTRIBUTING.md

@@ -10,6 +10,8 @@ Help is welcome, whether that's a one-line fix or owning a whole area. This page
 build it, how the code is laid out, the few rules that aren't negotiable, and where help is
 needed most.
 
+Questions, ideas or want to say what you're working on? Join the [Discord](https://discord.gg/FtU4NQNVDH).
+
 ## Get it running (about 10 minutes on a fast machine)
 
 You need stable Rust 1.95 or newer (required by egui/eframe 0.36). For the web