Docs improvement: install permissions

[danshearer]

(it seems better to use Discussions first, rather than raising an issue)

The directory 'userfiles' needs write permission by the user the webserver is running as, eg www-data. Otherwise operations such as changing the profile picture fail with an error in the errorlog and the app returns to the authentication screen.

For the stable distribution file, it would be a good idea to include an installation step of "chown -R root:www-data" or equivalent, rather than continuing with the id in the tar file, which could map to anything on the user's server.