splits uaa-docker into its own section, beefs up readme

[jduss4]

Changes proposed in this pull request:

• sets up basic containerization needed to run a UAA server locally
• provides documentation about the basic purpose of UAA

Originally the UAA docker directory was pushed along with the example cloud app. I have decoupled them for now, although the example app can certainly be configured to work with this UAA instance.

Things to check

• does the documentation make sense? Is it missing anything that would help you understand what it is for?
• are the steps "good enough" for us at this moment? For example, needing Zscaler to be disabled to set it up

Security considerations

• because this is being run locally, the only security considerations would be to your own machine by disabling zscaler temporarily. This could be mitigated by incorporating the Zscaler certificate into the Docker images, something which I have not pursued in the interests of making progress instead of getting bogged down in details

[jduss4]

Note: the PRIVATE KEY in the uaa.yml file is coming from the example file that I found in cloud foundry's own repo: https://raw.githubusercontent.com/cloudfoundry/uaa/4.27.0/uaa/src/main/resources/required_configuration.yml

[echappen]

LGTM, thanks for setting this up. If we choose this path for more dev work, then I'd consider incorporating the Zscaler cert into the routine. But for now, this is a fine way to get the hang of UAA.