Added SDL migration information to the top level README.md

Thanks for the feedback!
https://github.com/exult/exult/issues/379#issuecomment-1701066992

README.md

@@ -10,7 +10,8 @@ emulators, and popular games including Valve's award winning catalog
 and many Humble Bundle games.
 
 More extensive documentation is available in the docs directory, starting
-with README.md
+with [README.md](docs/README-android.md). If you are migrating from SDL 3.0
+from SDL 2.0, the changes are extensively documented in [README-migration.md](docs/README-migration.md).
 
 Enjoy!