Update NEWS for MPX support in the dynamic linker

[Philippe Waroquiers]

On Fri, 2014-08-08 at 11:24 -0700, Roland McGrath wrote:
> > I don't know. But changing "...affect Valgrind versions prior to 3.9" to
> > "...affect Valgrind versions up to 3.9" should be enough.
> 
> Indeed.  But it's nicer if we can say clearly (and accurately) when it will
> be fixed.
The next release will be called 3.10.0
The plan is:
  code freeze end of Aug
  release the 10 Sept
Philippe

NB: like all plans, it might change :)