message: added docstring to builder
diff: @@ -138,6 +138,9 @@ impl<'a> ExtractorBuilder<'a> {
  self
  }
  + /// finalize configuration of ExtratorBuilder and return an Extractor
  + ///
  + /// requires either with_url or with_response to have been used in the build process
  pub fn build(&self) -> Result<Extractor<'a>> {
  if self.url.is_empty() && self.response.is_none() {
  bail!("Extractor requires either a URL or a FeroxResponse be specified")
  