Add README for conformance test
Add README for conformance test

Signed-off-by: Yiqiao Pu <ypu@redhat.com>

Closes: #965
Approved by: rhatdan

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