Describe IDL in VERSIONED_API_README.md

Developers working with the Versioned API should know how to use IDL fields such as api_version, strict, and reply_type. (And is there a way in IDL to deprecate a command in an API version?) We should also explain that an API v1 command's IDL definition generates a C++ base class, and how developers should inherit from that base class. Let's add a bit more detail to VERSIONED_API_README.md.