admin stuff on rdf:PlainLiteral

* I added myself and Peter as editors (pursuant to a decision by the 
  chairs and other editors)

* I revised the acknowledgements section to thank many of you who have
  been helpful.   (If you were, and I forgot you, please let me know.)

* I added a "Changes Since Last Call" section (needed for CR), and
  populated it with what I could think of.  Help here is welcome.

     -- Sandro