installation.md: setcap on SFTPGo

DOCS/installation.md

@@ -255,6 +255,8 @@ Create a directory for configuration: `mkdir /etc/sftpgo`
 
 Copy the systemd service: `cp /install/sftpgo.service /etc/systemd/system/sftpgo.service`
 
+Allow listening on privileged ports: `setcap 'cap_net_bind_service=+ep' /usr/local/bin/sftpgo`
+
 #### Configure SFTPGo for ServNest
 
 Generate a key pair using `ssh-keygen -f /etc/sftpgo/ed25519 -t ed25519 -N "" -C ""`