Add Clamd hint

docs/prerequesite-system.md

@@ -13,12 +13,14 @@ Before you run **mailcow: dockerized**, there are a few requirements that you sh
 Please make sure that your system has at least the following resources:
 
 | Resource                | mailcow: dockerized              |
-| ----------------------- | --------------------- |
+| ----------------------- | -------------------------------- |
 | CPU                     | 1 GHz                            |
-| RAM                     | 1 GiB                 |
+| RAM                     | 1 GiB (or better 1,5 GiB + Swap) |
 | Disk                    | 5 GiB (without emails)           |
 | System Type             | x86_64                           |
 
+ClamAV is a greedy RAM muncher. You can disable it in `mailcow.conf` by settings SKIP_CLAMD=y.
+
 ## Firewall & Ports
 
 Please check if any of mailcow's standard ports are open and not in use by other applications: