added feature vs addendum

added feature

addendum

noun
  • The height by which the tooth of a gear projects beyond (outside for external, or inside for internal) the standard pitch circle or pitch line. 

  • Something to be added; especially text added as an appendix or supplement to a document. 

  • A postscript. 

How often have the words added feature and addendum occurred in a corpus of books? (source: Google Ngram Viewer )