update install instructions for Debian, Raspbian and Ubuntu
update install instructions for Debian, Raspbian and Ubuntu

Resolves: #1918

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

Closes: #2088
Approved by: rhatdan

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