Add additional recommendations to upgrading docs

[tcdowney]

Add some recommendations around backing up the old tas-adapter-values.yml and cluster state before upgrading so that operators can have an easier time recovering when something goes wrong.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.