[README] Warn about root installation issues

[VWoeltjen]

Mitigates #1151.

Author Checklist

1. Changes address original issue? Partially *
2. Unit tests included and/or updated with changes? N (documentation only)
3. Command line build passes? Yes
4. Changes have been smoke-tested? Yes

* Original issue primarily effects developers building locally and using the dev server; issue is not addressed, but a workaround/warning is conveyed to that audience

[akhenry]

Reviewer Checklist

1. Changes appear to address issue? Y
2. Appropriate unit tests included? N/A - no code change
3. Code style and in-line documentation are appropriate? N/A - no code change
4. Commit messages meet standards? Y