added ssh precisions

tutos/server.md

@@ -225,6 +225,9 @@ gateway : fe80::226:bff:feef:59ff
 	- **ssh-copy-id -o PreferredAuthentications=password -i /path/to/key.pub <username>@<server_ip>**
 - first time you authenticate by key :
 	- **ssh -i /path/to/key <username>@<server_ip>**
+- if next times you still need it ?
+	- check if key is loaded into agent : **ssh-add -l**
+	- if not, add it : **ssh-add ~/.ssh/your_key**
 
 
 ## list user : **cat /etc/passwd**