explain what a netmask is

README.md

@@ -127,6 +127,8 @@ On the `pg_hba.conf` file add a line like that at the end:
 host    all             all             <the netmask you want to add>            scram-sha-256
 ```
 
+The netmask should look like `A.B.C.D/E` like `0.0.0.0/0` for everyone, for example.
+
 Also change the `listen_addresses` instruction to `'*'` in `postgresql.conf`.
 
 ## Install Nginx