[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>