[doc] document why to use sudoer to run intsallation scripts

related to:

- https://gist.github.com/dalf/60450e19b042bd39b9ce470c095d89b3#gistcomment-3263827
- https://github.com/searx/searx/issues/2013#issuecomment-647132313
- https://github.com/searx/searx/issues/2017#issuecomment-647700327

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/installation.rst

@@ -33,6 +33,13 @@ The following will install a setup as shown in :ref:`architecture`.  First you
 need to get a clone.  The clone is only needed for the installation procedure
 and some maintenance tasks (alternatively you can create your own fork).
 
+For the installation procedure, use a *sudoer* login to run the scripts.  If you
+install from ``root``, take into account that the scripts are creating a
+``searx``, a ``filtron`` and a ``morty`` user.  In the installation procedure
+these new created users do need read access to the clone of searx, which is not
+the case if you clone into a folder below ``/root``.
+
+
 .. code:: bash
 
    $ cd ~/Downloads