Improve ThreeMigrationGuide.md (#1878)

* Add `ASImageCacherCompletion` breaking API change.

* Remove unnecessary trailing pounds.

Author: rogerluan

ThreeMigrationGuide.md

@@ -1,3 +1,23 @@
-### Texture 3.0 Migration Guide ###
+## Texture 3.0 Migration Guide
 
-Got a tip for upgrading? Please open a PR to this document!
+Got a tip for upgrading? Please open a PR to this document!
+
+
+### Breaking API Changes
+
+`ASImageCacherCompletion` typedef has a new parameter: `ASImageCacheType cacheType`. Example:
+
+
+```swift
+ASPINRemoteImageDownloader.shared().cachedImage(with: url, callbackQueue: .main) { result in
+    …
+}
+```
+
+Becomes
+
+```swift
+ASPINRemoteImageDownloader.shared().cachedImage(with: url, callbackQueue: .main) { result, cacheType in
+    …
+}
+```