Add documentation for system registry keys

[fhunleth]

This is for all of the config and state keys involved with networking. I suspect that it's best to include keys provided by nerves_network_interface and nerves_wpa_supplicant for completeness since it will be annoying to jump around projects for the total picture. However, I'm open to other thoughts on handling this.

[ConnorRigby]

this doc has links to this with all the system registry keys.