Add instructions to use global shortcut in ubuntu

I'm a huge fan of your tool and have recently installed it on Ubuntu. For future users, I think adding some instructions to use global shortcuts in Ubuntu (same as with KDE Plasma) would be great. [This StackExchange answer](https://askubuntu.com/questions/1036473/ubuntu-18-how-to-change-screenshot-application-to-flameshot) is a pretty good start:

> If you need to replace the PrtScr shortcut do the following:
>
> 1. Release the PrtScr binding by this command
>
> `gsettings set org.gnome.settings-daemon.plugins.media-keys screenshot ''`
>
> 2. Go to Settings -> Devices -> Keyboard and scroll to the end. Press + and you will create custom shortcut.
>
> Enter name: "flameshot", command: /usr/bin/flameshot gui.
>
> 3. Set shortcut to PrtScr (print).
>
> That is it. Next time you push PrtScr flameshot will be launched.

It's trivial enough but I'm happy to provide a PR if useful.