Issue 506: Document a possible solution to prevent metadata key enumeration problems.

[kaladay]

Description

Treat the referenced problem as a configuration problem.
Provide documentation in the application.yml to help avoid the situation.

Fixes #506

Type of change

• This change is a documentation update.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• No testing needed.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[coveralls]

Coverage: 45.229% (-0.02%) from 45.252% when pulling 34c65fb on 506-duplicate_key-metadatum-documentation into 6119eb9 on staging.

[jcreel]

Succinct and clear. The two relevant points in the configuration reference each other for convenience.