title: R: add title to wordcloud graphics / png
body: <p>I have some working R code that generates a tag cloud from a term-document matrix. </p>
  
  <p>Now I want to create a whole bunch of tag clouds from many documents, and to inspect them visually at a later time. 
  To know which document(s)/corpus the tag-cloud picture belongs to, I'd lke to add a title to the generated graphic. How do I do that? </p>
  
  <p>Maybe this is obvious, but I'm still a beginner with R graphics.</p>
  
  <p>My own corpus is too big to list it here, but the code from this SO question (combined with the code form the accepted answer from SO user Andrie can be used:
  <a href="https://stackoverflow.com/questions/8069571/spaces-in-wordcloud">Spaces in wordcloud</a>
  I want to add a custom title and some more custom text to a picture like <a href="https://i.stack.imgur.com/ziPvP.png" rel="noreferrer">this</a></p>