Create a migration guide for changes to ProcessContext.

Author: andriyDev

release-content/migration-guides/process_trait_changes.md

@@ -0,0 +1,30 @@
+---
+title: Changes to the `Process` trait.
+pull_requests: []
+---
+
+`ProcessContext` no longer includes `asset_bytes`. This has been replaced by `asset_reader`. To
+maintain current behavior in a `Process` implementation, you can read all the bytes into memory.
+If previously, you did:
+
+```rust
+// Inside `impl Process for Type`
+let bytes = context.asset_bytes();
+// Use bytes here!
+```
+
+Then now, it should be:
+
+```rust
+// Inside `impl Process for Type`
+let reader = context.asset_reader();
+let mut bytes = vec![];
+reader
+    .read_to_end(&mut bytes)
+    .await
+    .map_err(|err| ProcessError::AssetReaderError {
+        path: context.path().clone_owned(),
+        err: err.into(),
+    })?;
+// Use bytes here!
+```