Add instructions for changing firewall via Command Line

[andygunn]

Sometimes users can't access the wireless AP to access the Admin interface - and you can also "lock yourself out" of the admin panel if it is only available on the WAN interface.

Add some documentation to edit /etc/config/firewall via the Command line to resolve this:

Copy the entry that looks like:

config rule
        option src 'lan'
        option dest_port '443'
        option target 'ACCEPT'
        option proto 'tcp'

Paste the entry and change "lan" to "wan".