[isabelle-dev] NEWS

[Makarius]

* System: Experimental support for multithreading, using Poly/ML 5.1 
  (internal version from CVS). The maximum number of threads is specified 
  via usedir option -M or the "max-threads" setting in Proof General. 
  User-code needs to observe certain guidelines for thread-safe 
  programming, see appendix A in the Isar Implementation manual.