Create a migration guide for Process trait changes.

Author: andriyDev

release-content/migration-guides/process_trait_changes.md

@@ -0,0 +1,55 @@
+---
+title: The `Process` trait no longer has a single output.
+pull_requests: []
+---
+
+In previous versions, the `Process` trait had an associated type for the output loader, and took a
+`writer` argument. This is no longer the case (in order to support one-to-many asset processing).
+This change requires that users indicate whether they are using the "single" writer mode or the
+"multiple" writer mode.
+
+If your previous trait implementation of `Process` looked like this:
+
+```rust
+impl Process for MyThing {
+    type Settings = MyThingSettings;
+    type OutputLoader = SomeLoader;
+
+    async fn process(
+        &self,
+        context: &mut ProcessContext<'_>,
+        meta: AssetMeta<(), Self>,
+        writer: &mut Writer,
+    ) -> Result<SomeLoader::Settings, ProcessError> {
+        // Write to `writer`, then return the meta file you want.
+        let meta = todo!();
+        Ok(meta)
+    }
+}
+```
+
+Now it needs to look like:
+
+```rust
+impl Process for MyThing {
+    type Settings = MyThingSettings;
+
+    async fn process(
+        &self,
+        context: &mut ProcessContext<'_>,
+        meta: AssetMeta<(), Self>,
+        writer_context: WriteContext<'_>,
+    ) -> Result<(), ProcessError> {
+        let writer = writer_context.write_single().await?;
+        // Write to `writer`, then return the meta file you want.
+        let meta = todo!();
+        writer.finish(meta).await
+    }
+}
+```
+
+In addition, the returned `writer` is a wrapper around `&mut Writer`. This means you may need to
+explicitly dereference in order to pass this writer into functions that take a `&mut Writer`.
+
+This does not apply if you are using the `LoadTransformAndSave` process - existing uses should
+continue to work.