README: Add a blurb about FreeBSD 11.1

The clang 4.0 compiler that ships with FreeBSD 11.1 doesn't
work well with hwloc (older than 1.11.7), so if the following conditions hold

- one is building a version of Open MPI to include the internal hwloc package
- and the version of Open MPI's internal hwloc package is older than 1.11.7
- using the clang 4.0 compiler that ships with this release of FreeBSD

then one may observe segfaults, floating pointer exceptions, etc.

One workaround is to use the GNU compiler.

Related to #3992.
[skip ci]

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

README

@@ -136,6 +136,11 @@ General notes
     with Oracle Solaris Studio 12.5
   - OpenBSD.  Requires configure options --enable-mca-no-build=patcher
     and --disable-slopen with this release.
+  - There is a known issue with FreeBSD 11.1 and the clang-4.0 system
+    compiler that prevents proper compilation of older hwloc packages
+    (1.11.6 and older) shipped with some Open MPI releases. This problem
+    can be avoided by using the GNU compiler to build Open MPI, or by
+    using an external hwloc (1.11.7 or newer).
 
 Compiler Notes
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