Add example for how to listen for network updates

[fhunleth]

While this is technically a system_registry thing, It feels really helpful to anyone using the library. Here's an example:

https://github.com/fhunleth/nerves_init_gadget/blob/v0.1.0/lib/nerves_init_zero/network_manager.ex#L16
https://github.com/fhunleth/nerves_init_gadget/blob/v0.1.0/lib/nerves_init_zero/network_manager.ex#L20-L29

[ConnorRigby]

here are some docs