Create a migration guide.

Author: andriyDev

release-content/migration-guides/asset_watcher_async_sender.md

@@ -0,0 +1,12 @@
+---
+title: AssetSources now give an `async_channel::Sender` instead of a `crossbeam_channel::Sender`
+pull_requests: []
+---
+
+Previously, when creating an asset source, `AssetSourceBuilder::with_watcher` would provide users
+with a `crossbeam_channel::Sender`. Now, this has been changed to `async_channel::Sender`.
+
+If you were previously calling `sender.send(AssetSourceEvent::ModifiedAsset("hello".into()))`, now
+it would be `sender.send_blocking(AssetSourceEvent::ModifiedAsset("hello".into()))`. These channels
+are very comparable, so finding an analogous method between `crossbeam_channel` and `async_channel`
+should be straight forward.