Add PPA/command line instructions to GNU/Linux Download section

[GreenXenith]

Current Linux links take you to places like https://packages.ubuntu.com/search?keywords=minetest which are really vague and don't make much sense to a lot of people. It would be a huge help to put this at the bottom of the Linux portion (specifically a Debian/Ubuntu portion?):

sudo add-apt-repository ppa:minetestdevs/stable
sudo apt-get update
sudo apt-get install minetest

I get many people asking how to get the latest version as there are no good places to get these simple instructions from.