Merge pull request #435 from HDFGroup/chogan/mpiio_doc

Clarify MPI requirements in README

README.md

@@ -11,7 +11,9 @@ Hermes is a heterogeneous-aware, multi-tiered, dynamic, and distributed I/O buff
 * [Thallium](https://mochi.readthedocs.io/en/latest/installing.html) - RPC library for HPC. Use a version greater than `0.5` for RoCE support.
 * [GLOG](https://github.com/google/glog) - The Google logging library.
 * [GLPK](https://www.gnu.org/software/glpk/) - GNU Linear Programming Kit
-* MPI (tested with MPICH `3.3.2` and OpenMPI `4.0.3`).
+* MPI (tested with MPICH `3.3.2` and OpenMPI `4.0.3`). Note: The MPI-IO adapter
+  only supports MPICH. If you don't need the MPI-IO adapter you can use OpenMPI,
+  but you must define the CMake variable `HERMES_ENABLE_MPIIO_ADAPTER=OFF`.
 * The [Catch2](https://github.com/catchorg/Catch2) testing framework (only required if built with `-DBUILD_TESTING=ON`)
 
 ## Building