Add experimental/zfs-on-coreos to docs/index.rst toctree

This fixes a warning that the buildbot had:

checking consistency...
/srv/buildslave/flocker-docs/build/docs/experimental/zfs-on-coreos.rst::
WARNING: document isn't included in any toctree

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>