docs: document the initializer

Author: ctron

site/content/advanced.md

@@ -29,3 +29,46 @@ And then, define the entrypoint in your `lib.rs` like (does not need to be `asyn
 #[wasm_bindgen(start)]
 pub async fn run() {}
 ```
+
+## Initializer
+
+Since: `0.19.0-alpha.1`.
+
+Trunk supports tapping into the initialization process of the WASM application. By
+default, this is not active and works the same way as with previous versions.
+
+The default process is that trunk injects a small JavaScript snippet, which imports the JavaScript loader generated
+by `wasm_bindgen` and calls the `init` method. That will fetch the WASM blob and run it.
+
+The downside with is, that during this process, there's no feedback to the user. Neither when it takes a bit longer to
+load the WASM file, nor when something goes wrong.
+
+Now it is possible to tap into this process by setting `data-initializer` to a JavaScript module file. This module file
+is required to (default) export a function, which returns the "initializer" instance. Here is an example:
+
+```javascript
+export default function myInitializer () {
+  return {
+    onStart: () => {
+      // called when the loading starts
+    },
+    onProgress: ({current, total}) => {
+      // the progress while loading, will be called periodically.
+      // "current" will contain the number of bytes of the WASM already loaded
+      // "total" will either contain the total number of bytes expected for the WASM, or if the server did not provide
+      //   the content-length header it will contain 0.
+    },
+    onComplete: () => {
+      // called when the initialization is complete (successfully or failed)
+    },
+    onSuccess: (wasm) => {
+      // called when the initialization is completed successfully, receives the `wasm` instance
+    },
+    onFailure: (error) => {
+      // called when the initialization is completed with an error, receives the `error`
+    }
+  }
+};
+```
+
+For a full example see: <https://github.com/trunk-rs/trunk/examples/initializer>.