Add more documentation (#166)

src/main.rs

@@ -1,3 +1,16 @@
+//! # iamb
+//!
+//! The iamb client loops over user input and commands, and turns them into actions, [some of
+//! which][IambAction] are specific to iamb, and [some of which][Action] come from [modalkit]. When
+//! adding new functionality, you will usually want to extend [IambAction] or one of its variants
+//! (like [RoomAction][base::RoomAction]), and then add an appropriate [command][commands] or
+//! [keybinding][keybindings].
+//!
+//! For more complicated changes, you may need to update [the async worker thread][worker], which
+//! handles background Matrix tasks with [matrix-rust-sdk][matrix_sdk].
+//!
+//! Most rendering logic lives under the [windows] module, but [Matrix messages][message] have
+//! their own module.
 #![allow(clippy::manual_range_contains)]
 #![allow(clippy::needless_return)]
 #![allow(clippy::result_large_err)]
@@ -200,6 +213,7 @@ fn setup_screen(
     return Ok(ScreenState::new(win, cmd));
 }
 
+/// The main application state and event loop.
 struct Application {
     /// Terminal backend.
     terminal: Terminal<CrosstermBackend<Stdout>>,