Add supported desktop environments in readme

[archisman-panigrahi]

I have recently realized that Wayland on screen keyboards are desktop environment specific, and one keyboard does not work everywhere. Can you please mention in the README where sysboard works?

[System64fumo]

OSKs aren't desktop environment specific on wayland?
They should just work on most if not all wayland compositors..?
Is there any specific compositor you've tested in which OSKs don't work?