README: Add a blurb about FreeBSD 11.1

The clang 4.0 compiler that ships with FreeBSD 11.1 doesn't
work well with OpenMPI.  Workaround is to use a GNU compiler.

Related to #3992.
[skip ci]

Signed-off-by: Howard Pritchard <howardp@lanl.gov>

README

@@ -136,6 +136,9 @@ General notes
     with Oracle Solaris Studio 12.5
   - OpenBSD.  Requires configure options --enable-mca-no-build=patcher
     and --disable-slopen with this release.
+  - Problems have been reported when building Open MPI on FreeBSD 11.1
+    using the clang-4.0 system compiler. A workaround is to build
+    Open MPI using the GNU compiler.
 
 Compiler Notes
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