Add release note about PGI compilers.

This commit was SVN r9459.

README

@@ -189,6 +189,10 @@ base as of this writing (27 Feb 2006):
   eliminate the extra overhead of software MPI message matching where
   possible.
 
+- The Portland Group compilers seem to have a signed/unsigned casting
+  problem that prevents some of the Intel MPI test suite from
+  completing properly.  The problem has been reported to PGI.
+
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 Building Open MPI