Add a "Known issues" section, and the connectx XRC + message

coalescing bug.

This commit was SVN r20094.

README

@@ -132,6 +132,16 @@ Detailed Open MPI v1.3 Feature List:
     - Many other small improvements and bug fixes, too numerous to
       list here
 
+Known issues
+------------
+
+  o ConnectX XRC support does not work properly with message
+    coalescing.  If you attempt to use both of them together, Open MPI
+    will issue a warning and disable message coalescing.
+
+  o Please also see the Open MPI bug tracker for bugs beyond version
+    1.3: https://svn.open-mpi.org/trac/ompi/report.
+
 ===========================================================================
 
 The following abbreviated list of release notes applies to this code