Document digitalocean FLOC-985

Fixes FLOC-985 - https://clusterhq.atlassian.net/browse/FLOC-985

Note that you can't actually follow the kernel upgrade part of the instructions until DigitalOcean addresses a support ticket I opened. Possibly a kernel upgrade is unnecessary, we need to figure out if unupgraded version (3.13-9 from April) has problems with ZFS.
A start of this is https://github.com/ClusterHQ/flocker/pull/976/files#diff-8a73f42af2694c601b41bb2f65e3992aR116