add faq entry for 401 errors for users who do not load test data

Add a FAQ entry to address possible issue for users who do not load test_data. If Slumber settings are not updated with proper database credentials the server will give a 401 error::
    
    "POST /api/v1/version/ HTTP/1.1" 401 0
    (Build) [readthedocsorg:] Exception in creating version: Client Error 401:http://localhost:8000/api/v1/version/

Updated the doc with instructions to change SLUMBER_USERNAME and SLUMBER_PASSWORD if test_data is not loaded.