Extend readme

Author: oxkitsune

examples/rust/custom_callback/README.md

@@ -6,16 +6,6 @@ thumbnail_dimensions = [480, 291]
 
 Example showing how to control an app with the Rerun viewer, by extending the viewer UI.
 
-The example builds on the [`extend_viewer_ui`](../extend_viewer_ui/) example, but is still rather basic.
-
-## Architecture
-
-```mermaid
-graph LR;
-    App(App) --> Viewer(Viewer);
-    Viewer(Viewer) --> App(App);
-```
-
 <picture>
   <img src="https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/full.png" alt="Custom Viewer Callback example screenshot">
   <source media="(max-width: 480px)" srcset="https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/480w.png">
@@ -24,9 +14,23 @@ graph LR;
   <source media="(max-width: 1200px)" srcset="https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/1200w.png">
 </picture>
 
-[#2337](https://github.com/rerun-io/rerun/issues/2337): Note that in order to spawn a web viewer with these customizations applied,
-you have to build the web viewer of the version yourself.
-This is currently not supported outside of the Rerun repository.
+> [!NOTE]
+> [#2337](https://github.com/rerun-io/rerun/issues/2337): In order to spawn a web viewer with these customizations applied, you have to build the web viewer of the version yourself. This is currently not supported outside of the Rerun repository.
+
+## Overview
+
+This example is divided into two parts:
+
+The example is divided into two parts:
+
+- **Viewer** ([`src/viewer.rs`](src/viewer.rs)): Wraps the Rerun viewer inside an [`eframe`](https://github.com/emilk/egui/tree/master/crates/eframe) app.
+- **App** ([`src/app.rs`](src/app.rs)): The application that uses the Rerun SDK.
+
+In the `app`, an additional TCP listener is opened to allow the `viewer` to send messages to the `app`.
+Similar to the [`extend_viewer_ui`](../extend_viewer_ui/) example, the `viewer` is wrapped in an `eframe` app, which allows us to handle the extra communication logic and define our own contorl UI using [`egui`](https://github.com/emilk/egui).
+
+The communication between the `viewer` and the `app` is implemented in the [`comms`](src/comms/) module. It defines a simple protocol to send messages between the `viewer` and the `app` using [`bincode`](https://github.com/bincode-org/bincode).
+The protocol supports really basic commands that the `viewer` can send to the `app`, such as logging a [`Boxes3D`](https://www.rerun.io/docs/reference/types/archetypes/boxes3d) or [`Point3D`](https://www.rerun.io/docs/reference/types/archetypes/points3d) to an entity, or changing the radius of a set of points that is being logged.
 
 ## Testing it