document on encryption at rest

[ddiabe]

🗣 Description

💭 Motivation and context

📷 Screenshots (DELETE IF UNAPPLICABLE)

🧪 Testing

✅ Pre-approval checklist

• There is a gitIssue that this PR resolves
• Git Issue that this PR solves has been selected in the Development section
• The PR's base branch has been modified to be the proper branch.
• Changes are limited to a single goal AND
  the title reflects this in a clear human readable format
• Issue that this PR solves has been selected in the Development section
• I have read and agree to LME's CONTRIBUTING.md document.
• The PR adheres to LME's requirements in RELEASES.md
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.

✅ Pre-merge Checklist

• All tests pass.
• PR has been tested and the documentation for testing is above.
• Squash and merge all commits into one PR level commit.

✅ Post-merge Checklist

• Delete the branch to keep down number of branches.
• The PR is labeled with feat for an added new feature, update for an update, OR fix for a fix.
• The PR contains Resolves #<issue #> so that merging it closes out the corresponding issue. For example Resolves #132.