code: public ConfigurationSourcesModuleBuilder addSource(ConfigurationSource configurationSource) {
      if (configurationSource == null) {
        throw new NullPointerException("configurationSource: null");
      }
      configurationSources.add(configurationSource);
      return this;
    }
docs: Adds the given {@code configurationSource} to the list.
  
  @param configurationSource configuration source from which Configuration will be built.
  @return the current instance of the {@link ConfigurationSourcesModuleBuilder}.