add documentation for piwigo auto-login

Author: zisoft

content/preferences-settings/security.md

@@ -47,5 +47,8 @@ ask before exporting in overwrite mode
 password storage backend to use
 : The backend to use for password storage. Options: “auto” (default), “none”, “libsecret”, “kwallet”, "apple_keychain" (on macOS), "windows_credentials" (on Windows). 
 
+auto login to storage server
+: When exporting images to an external storage server (i.e. Piwigo) this option allows to automatically perform a login to the server. A password storage backend is required to be set.
+
 executable for playing audio files
 : Define an external program which is used in the lighttable view to play audio files that some cameras record to keep notes for images (default “aplay”).