Flatpak shown in README.md

[Logster9982]

Added the newly released Flatpak installation method on the GitHub with a recommendation for steam deck users to install with said method.

[Logster9982]

Cannot edit wiki pages. Please add to Installation Instructions above the Ubuntu installation the following:

Universal/Steam Deck

For a universal installation in all Linux Distributions including the Steam Deck, please install via a Flatpak Installation

1. Follow instructions for your Distro to install Flathub
2. Download the AM2R Flatpak. This may be from your software center such as "Discover" on Steam Deck

If the installation succeeds but the game does not run, make sure to enable 32-bit support on your system with sudo dpkg --add-architecture i386.

[Miepee]

Fixed up some wording and added the flatpak instructions to the wiki as well.

[Logster9982]

Committed fixes