Add Clamd hint

docs/prerequesite-system.md

@@ -12,12 +12,14 @@ 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                 |
-| Disk                    | 5 GiB (without emails)|
-| System Type             | x86_64                |
+| Resource                | mailcow: dockerized              |
+| ----------------------- | -------------------------------- |
+| CPU                     | 1 GHz                            |
+| 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