Add installation instructions for Ubuntu Software Center

[googlymog]

Expected Behavior

On the installation instructions page add a section about installing from the Ubuntu Software Center. I am on 22.04 Jammy and Gourmand is available there. I couldn't get the flatpak or wheel installations to work unfortunately.

Current Behavior

New users would know they have the option to get a one click install that works. Right now, they might not realize this.

Possible Solution

No code needs to be added, just documentation. I'm not sure which versions of Ubuntu Gourmand will show up in, or how the Ubuntu software center works.

Environment

I am running Ubuntu 22.04.