Document (lack of) standardized approach to persisting WiFi connection settings in README

[Qqwy]

From the #nerves slack:

fhunleth: As for persisting network settings, the nerves_network way was to do it yourself. The thought at the time was that application writers would know better how to do that, but that turned out to be a hurdle.

This is currently not mentioned in the README. I think it would be great to mention more clearly:

• That by default the runtime settings of nerves_network are ephemeral and are lost on restart.
• That this is not something that nerves_network provides for you in any way.
• That work is being done on VintageNet as alternative that has this functionality but that it is not finished yet.