Correct Arch package info

[alerque]

My suggest to fix the incorrect instructions noted in #76. It's hard to give a one liner because the official method is clone, build, then install. Helper tools exist but are not universal and there are several possible ones people may prefer. Usually if people have one they know how to use it (e.g. paru or yay).

[MestreLion]

That's a very welcome clarification, thanks!