First shot at HW requirements on the download page

It comes up from time to time on IRC so I think we should have a section about it next to the download.

I guesstimated the values judging by what's available here but that's for 0.6/0.7 and there is a lot of variation.

I could use ideas/help how to better specify the GPU requirements.

Not sure if we still support Windows XP.

OSX sierra probably still has the issues described at here.