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.
I think we probably want to structure these section of documentation as follows:

> Here are some ways to install flocker
> - vagrant [link]
> - DO [link]
> - AWS [link]
> - RS [link]

followed by the corresponding sections. Thus, the expected document flow isn't going to be coming from reading a different method of installing flocker, but rather from a ToC. The introduction here should reflect that.