Add runtime requirements section to README
Add runtime requirements section to README

Add details on kernel version, criu, snapshotters, and links to existing
sections for developers/criu details.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@linux.vnet.ibm.com>

mikebrow/containerd,miaoyq/containerd,crosbymichael/containerd,containerd/containerd,containerd/containerd,dmcgowan/containerd,dmcgowan/containerd,kunalkushwaha/containerd,kunalkushwaha/containerd,thaJeztah/containerd,thaJeztah/containerd,estesp/containerd,mikebrow/containerd,vdemeester/containerd,estesp/containerd,miaoyq/containerd,crosbymichael/containerd,vdemeester/containerd