add readme

Author: oxkitsune

examples/rust/custom_callback/README.md

@@ -0,0 +1,34 @@
+<!--[metadata]
+title = "Custom Viewer Callback"
+thumbnail = "https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/480w.png"
+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">
+  <source media="(max-width: 768px)" srcset="https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/768w.png">
+  <source media="(max-width: 1024px)" srcset="https://static.rerun.io/custom_callback/1434da408fd59ea1349169784b47d8ffc285022e/1024w.png">
+  <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.
+
+## Testing it
+
+First start the Rerun SDK app with `cargo run -p custom_callback --bin custom_callback_app`,
+and then start the extended viewer with `cargo run -p custom_callback --bin custom_callback_viewer`.