README: improve install and run instructions

[meonkeys]

This PR attempts to improve install and first-time setup instructions for new users. I much preferred the Debian package install, and I found this didn't work without adding the uniemoji input source in the keyboard settings.

This might only be an issue for 20.04 and later.

[salty-horse]

These are good additions, but I worry that they're specific to Ubuntu.

Can you mention that this is a Debian package?

I'm only familiar with Ubuntu, from which PPA originated. Will add-apt-repository ppa:helkaluin/ibus-uniemoji work on other Debian-based systems?

I'm assuming that "Settings → Keyboard" is also system-specific. For example, I'm using Gnome Flashback, and there it's called "System Settings".

[meonkeys]

I worry that they're specific to Ubuntu.

Yes, they are. I assumed ibus-uniemoji was Ubuntu-specific based on what I found in this Readme.

I mostly use Ubuntu, so I'm not the best person to help with cross-distro / cross-desktop documentation.

That said, I see that on my Ubuntu computer /usr/bin/apt-add-repository is provided by software-properties-common, and Debian has that... although I did think PPAs were Ubuntu-specific.