README: clarify OMPI's same-version requirements

Clarify that Open MPI requires the exact same version number in all
parts of an Open MPI / OSHMEM job for it to work properly.

README

@@ -1395,6 +1395,12 @@ Note that this definition encompasses several things:
   * mpirun / oshrun command line options
   * MCA parameter names / values / meanings
 
+However, this definition only applies when the same version of Open
+MPI is used with all instances of the runtime and MPI / OSHMEM
+processes in a single MPI job.  If the versions are not exactly the
+same everywhere, Open MPI is not guaranteed to work properly in any
+scenario.
+
 Software Version Number
 -----------------------
 
@@ -1426,6 +1432,8 @@ format.  Each of the three numbers has a specific meaning:
     user-observable changes or new features, only the third integer
     will be increased (e.g., from v4.3.0 to v4.3.1).
 
+The "A.B.C" version number may optionally be followed by a Quantifier:
+
   * Quantifier: Open MPI version numbers sometimes have an arbitrary
     string affixed to the end of the version number. Common strings
     include: