Readme doc updates

A variety of updates to the README. Two main goals with these updates:

- Be consistent in formatting
- Be consistent in using RSpec 3 syntax
It's in the upgrade doc cukes. From fe4f8123f commit message:

> Remove app upgrade notes in favor of linked docs.
>
> The feature notes have all of the details of how to properly upgrade.
> These are hosted online and were already linked in the above section.
> Remove the duplication as it detracts from the remainder of the README.