add notes to migration

Author: jderochervlk

pages/docs/manual/latest/migrate-to-v11.mdx

@@ -62,6 +62,12 @@ Open it so it's available in the global scope.
  }
 ```
 
+One major change to be aware of is that array access now returns an `option`.
+```res
+let firstItem = myArray[0] // Some("hello")
+```
+If you would like to not use an `option`, you can use [`Array.getUnsafe`](api/core/array#value-getUnsafe).
+
 For a detailed explanation on migration to ReScript Core, please refer to its [migration guide](https://github.com/rescript-association/rescript-core#migration). A semi-automated script is available as well.
 
 See ReScript Core API docs [here](api/core).