Update apptainer installation instructions

[Andrew-S-Rosen]

Closes #9.

The apptainer guide suggests:

sudo apt update
sudo apt install -y software-properties-common
sudo add-apt-repository -y ppa:apptainer/ppa
sudo apt update
sudo apt install -y apptainer

which worked for me. I'm not sure if you want the full block though. Right now, I linked to the official docs and included the missing sudo add-apt-repository -y ppa:apptainer/ppa. But one could argue that the full example above should be included.

[svandenhaute]

Well, since it's going to depend on the linux distribution and/or existing packages anyway, perhaps it's better to just include a link to the installation instructions. That way, if the instructions change, we're automatically up-to-date and don't need to worry about it (or about which statements to include and which are are considered trivial).

[Andrew-S-Rosen]

Makes sense to me! I agree.