Add "maintenace mode only" message at the top of README

[TimLariviere]

Since Xamarin.Forms is in maintenance mode only starting 5.0 and will go out of support in May 2024, we will do the same with Fabulous.XamarinForms.

We need to add a message at the top of the README file to make this clear to everyone.

https://dotnet.microsoft.com/en-us/platform/support/policy/xamarin