Improving the distro-independent installation guide

[sushimustwrite]

Per photo/frontend#791, all those guides will get hard to maintain soon. What happens when we change a specific system requirement? We'd have to update all those guides individually. Easier to have a single Linux guide.

Edit: Dur, we do have such a guide, though it could be improved. See #18 and #5

[jmathai]

Like this idea but how do we provide steps for apt vs yum vs something else? Or do we not?