Fix explanation of reason for upgrade.

We can install old devel packages, so document the real reason for upgrading
the kernel on AWS.
Change "below" to be a link to the instructions.
A bit of an introduction.
Merge pull request #976 from ClusterHQ/cleanup-installation-instructions-FLOC-997

Add a summary of flocker-node installation options.

Author: @tomprince 
Reviewer: @wallrj

Fixes https://clusterhq.atlassian.net/browse/FLOC-997