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.
Mmmm. Do we actually care if they're using a too-new kernel image?