Update documentation

[emersonthis]

The README.md does not include a lot of useful configuration information that is documented in the vets-api docs. In some respects that document has become a more authoritative resource than the docs in this repo.

Almost everything under Caching Multiple Responses is not specific to the vets-api project, and is likely of great use to many betamocks users. Same for Error Responses.

It's also probably worth weeding out some artifacts in the current README, such as base_urls which appears to have been replaced by base_uri in services_config.yml.