Add an RFC about providing an official WSL image

Add an RFC that defines specifications about providing an official WSL Arch Linux image.

This overall topic has already been discussed and mostly hashed out in the [related mail thread](https://lists.archlinux.org/archives/list/arch-dev-public@lists.archlinux.org/thread/73A4BK7YK4BJBVXGMN2I5CROQAWI53VZ/

) in the Arch-Dev-Public mailing list.

Signed-off-by: Robin Candau <antiz@archlinux.org>