mkinitcpio -p linux fails because of missing dependencies

[hippietilley]

I had to install linux, linux-firmware, and lvm2 before mkinitcpio -p linux would resolve as expected. I could add this to the documentation and issue a PR if you would like.

[ejmg]

feel free to make a PR and if it checks out for me, i'll gladly merge it!

small heads up: i may be deprecating this guide soon in preference for a newer, more up to date one, i'll be writing in a few months (maybe). i think the contribution will be welcomed/useful to others all the same regardless.