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.
The real reason it's there, is that I wanted to prove to myself that DO didn't auto-detect and boot the kernel inside the image....so I did `yum update kernel` and power cycled. Predictably, that failed.

Then when I attempted to install the latest DO supported kernel, I found that `yum` didn't allow what it considered a *downgrade*, and instead I found instructions on using `rpm` for the task.

I thought those might be worth documenting for the reasons I put in https://github.com/ClusterHQ/flocker/pull/974#issuecomment-63297253

But I'll remove them.