Document how to use upstream DNS

[nicopace]

When connected to an upstream connection, the DHCP server gives you DNS information.
Implement a way to choose it instead of a fixed one.

[nicopace]

que todos los nodos de una nube resuelvan a traves del DNS que obtuvo el nodo gateway por DHCP. Pero que tambien haya la posibilidad de usar los que estan en /etc/config/lime

[ilario]

We could even use the one suggested by the ISP by default, why do we set different ones? @p4u @G10h4ck @altergui

[ilario]

I tested that it's possible to use ISP upstream DNS servers just un-setting the custom ones, so replacing

        list resolvers '4.2.2.2'
        list resolvers '141.1.1.1'
        list resolvers '2001:470:20::2'

with

        list resolvers ''

in /etc/config/lime and running lime-config.
Just deleting the resolvers list from lime file doesn't work as it gets re-generated, to set it empty works.
I verified that the DNS server is the upstream one looking at logread | grep nameserver.
This needs to be documented, at least in lime-example, and implemented in web-ui.

[ilario]

Fixed by @aparcar in #284