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.
And it also isn't currently available as a kernel on DO.
```
yum update https://kojipkgs.fedoraproject.org/packages/kernel/3.16.6/203.fc20/x86_64/kernel--3.16.6-203.fc20.x86_64.rpm
```
will install the latest kernel currently available from DO, and should continue to work.