Fix spelling/links.
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 #975 from ClusterHQ/fix-doc-build-FLOC-996

Add a spelling override and fix a link

Author: @tomprince 
Reviewer: @wallrj

Fixes FLOC-996
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
Merge remote-tracking branch 'origin/master' into document-digitalocean-FLOC-985

Conflicts:
	docs/gettingstarted/installation.rst