Add docs for build tags used by cri.
Add docs for build tags used by cri.

Since CRI is compiled in with containerd, builders should be aware that
these buildtags are available.

Signed-off-by: Brian Goff <dc73d0991b982a8303e713ef531a313af77e242a@gmail.com>

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