Adds usage docs and no-sensitive-content warning to README

General MR

Summary

Adds basic "this is not application code, just some text files to be used for testing live reposync services against github+repo1 without breaking stuff" warnings.

Relevant logs/screenshots

See the content changes in the preview.

Linked Issue

Relates to reposync migration https://repo1.dso.mil/big-bang/team/tools/repo-sync/-/issues/192 and testing https://repo1.dso.mil/big-bang/team/tools/repo-sync/-/issues/294 stories

Upgrade Notices

N/A