If you want those files to be managed by collectstatic
Let's assume after running collectstatic, your favicon.ico file ends up being copied in a root subdirectory, located in your STATIC_ROOT directory.
Then, with:
WHITENOISE_ROOT = os.path.join(STATIC_ROOT, 'root')
Whitenoise will serve all files in STATIC_ROOT/root/ at the root of your application.
In your case, STATIC_ROOT/root/favicon.ico will be served at /favicon.ico.
If you don't want those files to be managed by collectstatic
You can have a root_staticfiles folder in your BASE_DIR which simply contains the static files you want to serve at /.
WHITENOISE_ROOT = os.path.join(BASE_DIR, 'root_staticfiles')
In this case, Whitenoise will serve all files in BASE_DIR/root_staticfiles/ at the root of your application.
Update about pathlib (2022-10-04)
Since a while now, the default settings.py Django creates uses pathlib. To be consistent with it, one may replace os.join calls with / operators, eg.:
WHITENOISE_ROOT = STATIC_ROOT / 'root'