Add migration guide and release notes.

Author: andriyDev

release-content/migration-guides/asset_saver_changes.md

@@ -0,0 +1,47 @@
+---
+title: SavedAsset now contains two lifetimes.
+pull_requests: []
+---
+
+`SavedAsset` now holds two lifetimes instead of one. This is primarily used in the context of
+`AssetSaver`. So previously, users may have:
+
+```rust
+impl AssetSaver for MySaver {
+    type Asset = MyAsset;
+    type Settings = ();
+    type OutputLoader = MyLoader;
+    type Error = std::io::Error;
+
+    async fn save(
+        &self,
+        writer: &mut Writer,
+        asset: SavedAsset<'_, Self::Asset>,
+        settings: &Self::Settings,
+    ) -> Result<(), Self::Error> {
+        todo!()
+    }
+}
+```
+
+Now with the extra `SavedAsset` lifetime:
+
+```rust
+impl AssetSaver for MySaver {
+    type Asset = MyAsset;
+    type Settings = ();
+    type OutputLoader = MyLoader;
+    type Error = std::io::Error;
+
+    async fn save(
+        &self,
+        writer: &mut Writer,
+        asset: SavedAsset<'_, '_, Self::Asset>,
+        settings: &Self::Settings,
+    ) -> Result<(), Self::Error> {
+        todo!()
+    }
+}
+```
+
+In practice, this should not have an impact on usages.

release-content/release-notes/asset_saving.md

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+---
+title: Asset Saving
+authors: ["@andriyDev"]
+pull_requests: []
+---
+
+Since Bevy 0.12, we've had the `AssetSaver` trait. Unfortunately, this trait was not really usable
+for asset saving: it was only intended for use with asset processing! This was a common stumbling
+block for users, and pointed to a gap in our API.
+
+Now, users can save their assets using `save_using_saver`. To use this involves two steps.
+
+### 1. Building the `SavedAsset`
+
+To build the `SavedAsset`, either use `SavedAsset::from_asset`, or `SavedAssetBuilder`. For example:
+
+```rust
+let asset_path: AssetPath<'static> = "my/file/path.whatever";
+let mut builder = SavedAssetBuilder::new(asset_server.clone(), asset_path.clone());
+
+let subasset_1 = Line("howdy".into());
+let subasset_2 = Line("goodbye".into());
+let handle_1 = builder.add_labeled_asset_with_new_handle(
+    "TheFirstLabel", SavedAsset::from_asset(&subasset_1));
+let handle_2 = builder.add_labeled_asset_with_new_handle(
+    "AnotherOne", SavedAsset::from_asset(&subasset_2));
+
+let main_asset = Book {
+    lines: vec![handle_1, handle_2],
+};
+let saved_asset = builder.build(&main_asset);
+```
+
+Note that since these assets are borrowed, building the `SavedAsset` should happen in the same async
+task as the next step.
+
+### 2. Calling `save_using_saver`
+
+Now, with a `SavedAsset`, we can just call `save_using_saver` and fill in any arguments:
+
+```rust
+save_using_saver(
+    asset_server.clone(),
+    &MyAssetSaver::default(),
+    &asset_path,
+    saved_asset,
+    &MySettings::default(),
+).await.unwrap();
+```
+
+Part of this includes implementing the `AssetSaver` trait on `MyAssetSaver`. In addition, this is an
+async function, so it is likely you will want to spawn this using `IoTaskPool::get().spawn(...)`.