Create release notes for runtime asset sources.

Author: andriyDev

release-content/release-notes/runtime_sources.md

@@ -0,0 +1,51 @@
+---
+title: Adding and removing asset sources at runtime
+authors: ["@andriyDev"]
+pull_requests: []
+---
+
+Custom asset sources are a great way to extend the asset system to access data from all sorts of
+sources, whether that be a file system, or a webserver, or a compressed package. Unfortunately, in
+previous versions, asset sources could **only** be added before the app starts! This prevents users
+from choosing their sources at runtime.
+
+For a concrete example, consider the case of an application which allows you to pick a `zip` file to
+open. Internally, a `zip` is its own little filesystem. Representing this as an asset source is
+quite natural and allows loading just the parts you need. However, since we couldn't previously add
+asset sources at runtime, this wasn't possible!
+
+Now you can add asset sources quite easily!
+
+```rust
+fn add_source_and_load(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+) -> Result<(), BevyError> {
+    let user_selected_file_path: String = todo!();
+
+    asset_server.add_source(
+        "user_directory",
+        &mut AssetSourceBuilder::platform_default(&user_selected_file_path, None)
+    )?;
+
+    let wallpaper = asset_server.load("user_directory://wallpaper.png");
+    commands.spawn(Sprite { image: wallpaper, ..Default::default() });
+
+    Ok(())
+}
+```
+
+Asset sources can also be removed at runtime, allowing you to load and unload asset sources as
+necessary.
+
+We've also changed the behavior of registering asset sources. Previously, you needed to register
+asset sources **before** `DefaultPlugins` (more accurately, the `AssetPlugin`). This was uninuitive,
+and resulted in limitations, like effectively preventing crate authors from registering their own
+asset sources (since crate plugins often need to come after `DefaultPlugins`). Now, asset sources
+need to be registered after `AssetPlugin` (and so, `DefaultPlugins`).
+
+## Limitations
+
+A limitation is that asset sources added after `Startup` **cannot be processed**. Attempting to add
+such a source will return an error. Similarly, removing a processed source returns an error. In the
+future, we hope to lift this limitation and allow runtime asset sources to be processed.