Add security notice and built-in auth link

[pamelafox]

Purpose

This PR makes it clearer that the app is, by default, publicly accessible.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe:

[achandmsft]

Looks good!