Create a migration guide for registering asset sources.

Author: andriyDev

release-content/migration-guides/registering_asset_sources.md

@@ -0,0 +1,95 @@
+---
+title: Registering asset sources
+pull_requests: []
+---
+
+In previous versions, asset sources had to be registered **before** adding the `AssetPlugin` (which
+usually meant before adding the `DefaultPlugins`). Now, asset sources must be registered **after**
+`AssetPlugin` (and in effect, after `DefaultPlugins`). So if you had:
+
+```rust
+App::new()
+    .register_asset_source("my_source",
+        AssetSource::build()
+            .with_reader(move || Box::new(todo!()))
+    )
+    .add_plugins(DefaultPlugins)
+    .run();
+```
+
+Now, it will be:
+
+```rust
+App::new()
+    .add_plugins(DefaultPlugins)
+    .register_asset_source("my_source",
+        AssetSourceBuilder::new(move || Box::new(todo!()))
+    )
+    .run();
+```
+
+Note: See also "Custom asset sources now require a reader" for changes to builders.
+
+In addition, default asset sources **can no longer be registered like custom sources**. There are
+two cases here:
+
+## 1. File paths
+
+The `AssetPlugin` will create the default asset source for a pair of file paths. Previously, this
+was written as:
+
+```rust
+App::new()
+    .add_plugins(DefaultPlugins.set(
+        AssetPlugin {
+            file_path: "some/path".to_string(),
+            processed_file_path: "some/processed_path".to_string(),
+            ..Default::default()
+        }
+    ));
+```
+
+Now, this is written as:
+
+```rust
+App::new()
+    .add_plugins(DefaultPlugins.set(
+        AssetPlugin {
+            default_source: DefaultAssetSource::FromPaths {
+                file_path: "some/path".to_string(),
+                // Note: Setting this to None will just use the default path.
+                processed_file_path: Some("some/processed_path".to_string()),
+            },
+            ..Default::default()
+        }
+    ));
+```
+
+## 2. Custom default source
+
+Users can also completely replace the default asset source to provide their own implementation.
+Previously, this was written as:
+
+```rust
+App::new()
+    .register_asset_source(
+        AssetSourceId::Default,
+        AssetSource::build()
+            .with_reader(move || Box::new(todo!()))
+    )
+    .add_plugins(DefaultPlugins);
+```
+
+Now, this is written as:
+
+```rust
+App::new()
+    .add_plugins(DefaultPlugins.set(
+        AssetPlugin {
+            default_source: DefaultAssetSource::FromBuilder(Mutex::new(
+                AssetSourceBuilder::new(move || Box::new(todo!()))
+            )),
+            ..Default::default()
+        }
+    ));
+```