add a note about removing @ccallable for apps

Author: KristofferC

docs/src/index.md

@@ -86,3 +86,4 @@ PackageCompiler 2.0 comes with a few breaking changes.
 - The `audit_app` function has been removed. It caught too few problems to be useful in practice.
 - The keyword `app_name` in `create_app` has been removed and replaced with a more flexible version.
   If you used `app_name="Foo"`, replace it with `executables=["Foo"=>"julia_main"]`.
+- The `@ccallable` in front of the entry point functions of apps should be removed. Failure to do so might lead to strange errors during creation of the app.