add section about soptware upgrades using apt-get

sysadmin.adoc

@@ -317,6 +317,29 @@ installation using the following command:
  zpool add <pool-name> log </dev/path_to_fast_ssd>
 
 
+System Software Updates
+-----------------------
+
+We provide regular package updates on all repositories. You can
+install those update using the GUI, or you can directly run the CLI
+command 'apt-get':
+
+ apt-get update
+ apt-get dist-upgrade
+
+NOTE: The 'apt' package management system is extremely flexible and
+provides countless of feature - see `man apt-get` or <<Hertzog13>> for
+additional information.
+
+You should do such updates at regular intervals, or when we release
+versions with security related fixes. Major system upgrades are
+announced at the https://forum.proxmox.com/[Forum]. Those announcement
+also contain detailed upgrade instructions.
+
+TIP: We recommend to run regular upgrades, because it is important to
+get the latest security updates.
+
+
 Network Configuration
 ---------------------