README.md: add useful links
README.md: add useful links

Fix issue 279

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

containerd/stargz-snapshotter,containerd/stargz-snapshotter,containerd/stargz-snapshotter,containerd/stargz-snapshotter