update installation doc to reflect current status
update installation doc to reflect current status

Signed-off-by: Lokesh Mandvekar <46b23e80209bb43857d25fd87bafe9be89c7d33a@fedoraproject.org>

nalind/buildah-1,projectatomic/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1