Describe validator list format

[mDuo13]

Sites such as vl.ripple.com serve a list of recommended validators in a format for rippled servers to consume and set their UNLs.

We'd like to document what this JSON format means, how to read it, and how to create a file matching this format, so that other parties than Ripple could (eventually) create their own validator registry and vouch for trusted validators. They could also make cool apps that showcase the current recommended UNL in nice ways. (Some of that has already been done.)

The document should contain prominent and explicit warnings about using random shady validator lists, because doing so introduces a risk that they could fork away from the main network. (The lists published by Ripple, Coil, and XRPLF should all be OK and have sufficient overlap to make forking impossible.)

[mDuo13]

FWIW, the Validator List Method response format mostly covers this.