Add instruction how to add ActivityWatch to autostart on Ubuntu

[tmotyl]

Ubuntu is one of the most popular Linux distributions for desktop users, so it would lower entry barrier if there were more detailed installation instructions for Ubuntu

[rasdani]

Since I just installed ActivityWatch and added it to autostart on my Ubuntu 22.04 LTS, I woud like to write up some instructions :)
I could write on the GUI way of adding it, which should lower the entry barrier indeed.

[rasdani]

#124