Document [json_serializable/]lib/builder.dart – set an example

[kevmoo]

https://github.com/dart-lang/json_serializable/blob/bfc6df1af0ec91abc25c9cf3bfc0d763721d843e/json_serializable/lib/builder.dart#L9

Should explain how this relates to build.yaml and pkg/build, etc

CC @natebosch @jakemac53

[kevmoo]

Fixed in 3eabbf2