README:md: Document dnsServers behaviour
README:md: Document dnsServers behaviour

Taken from fa3de6987e3bc82ab6d85038c637f1871e5269cb commit message

Change-type: patch
Signed-off-by: Alexis Svinartchouk <37ac5e111a9b2f779e373f78efa4f7678b93feb1@balena.io>

resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin