Document that MPI_FILE operations are not THREAD_MULTIPLE_SAFE

[jsquyres]

Per @nysal #1762, and per discussion with @edgargabriel, it sounds like we don't have much confidence in THREAD_MULTIPLE for MPI_FILE operations. So we need to document this in the README or somesuch.

[gpaulsen]

Is this true for both OMPIO and ROMIO IO components?