Changelog: mention that StarPU-MPI now works on Darwin.

ChangeLog

@@ -35,6 +35,7 @@ New features:
   - Manual mapping of commands on specific devices is now possible
 
 Changes:
+  * Fix StarPU-MPI on Darwin.
   * The FxT code can now be used on systems other than Linux.
   * Keep only one hashtable implementation common/uthash.h
   * The cache of starpu_mpi_insert_task is fixed and thus now enabled by