Add docs to demo

Author: emilk

demo/src/lib.rs

@@ -1,3 +1,7 @@
+//! Demo code for the [`egui_plot`], hosted at <https://emilk.github.io/egui_plot/>.
+//!
+//! Each push to `main` re-deploys the demo.
+
 #![warn(clippy::all, rust_2018_idioms)]
 
 mod app;