README: Add explanation of persistent collectives

Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>

README

@@ -717,6 +717,14 @@ Open MPI Extensions
 
 - The following extensions are included in this version of Open MPI:
 
+  - pcollreq: Provides routines for persistent collective
+    communication operations and persistent neighborhood collective
+    communication operations, which are proposed in the MPI Forum as
+    of June 2018.  The function names are prefixed with MPIX_ instead
+    of MPI_, like MPIX_Barrier_init, because they are not standardized
+    yet.  Future versions of Open MPI will switch to the MPI_ prefix
+    once the MPI Standard which includes this feature is published.
+    See their man page for more details.
   - affinity: Provides the OMPI_Affinity_str() routine on retrieving
     a string that contains what resources a process is bound to.  See
     its man page for more details.