code: public static String ofReaderGroupInScope(String scopeName, String readerGroupName) {
          Exceptions.checkNotNullOrEmpty(readerGroupName, "readerGroupName");
          return String.format("%s/%s", ofReaderGroupsInScope(scopeName), readerGroupName);
      }
docs: Creates a resource representation for use in authorization of actions pertaining to the specified reader group
  within the specified scope.
  
  @param scopeName the name of the scope
  @param readerGroupName the name of the reader group
  @return a string representing the specified reader group
  @throws NullPointerException if {@code scopeName} or {@code streamName} are null
  @throws IllegalArgumentException if {@code scopeName} or {@code streamName} are empty