code: public static Map<String, ReadHandler<?, ?>> readHandlerMap(Map<String, ReadHandler<?, ?>> customHandlers) {
          return new ReadHandlerMap(customHandlers);
      }
docs: Creates a read-only Map of String to ReadHandler containing
  default ReadHandlers with customHandlers merged in. Use this to
  build the collection of read handlers once, and pass it to
  repeated calls to TransitFactory.reader. This can be more
  efficient than repeatedly passing a map of custom handlers to
  TransitFactory.reader, which then merges them with the default
  handlers and/or looks them up in a cache each invocation.
  @param customHandlers a map of custom ReadHandlers to use in addition
  or in place of the default ReadHandlers
  @return a ReadHandlerMap