tweak documentation

README.rst

@@ -108,7 +108,7 @@ The list is a work in progress, please contact us
 (Cristian.Lalescu@ds.mpg.de) if the procedure fails at any step of the
 process.
 
-*Note*: the HDF5 library may require MPI standard v1, we haven't tested
+*Note*: the HDF5 1.8x library may require MPI standard v1, we haven't tested
 this in detail. Some default OpenMPI3 installations will not work.
 
 Detailed full installation instructions:
@@ -140,6 +140,10 @@ Detailed full installation instructions:
     folders, hence the options of specifying the details.
     See also lines 143 through 160 of `CMakeLists.txt`.
 
+    *If using openmpi* you may need to recompile it with the
+    `--enable-mpi1-compatibility` flag, in order for HDF5 to compile and
+    link (step 4).
+
 2.
     Make directory PREFIX on a local fast partition.