README: Update with information about Reproducible Builds.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

README

@@ -140,6 +140,15 @@ General notes
     using the clang-4.0 system compiler. A workaround is to build
     Open MPI using the GNU compiler.
 
+- Open MPI has taken some steps towards Reproducible Builds
+  (https://reproducible-builds.org/).  Specifically, Open MPI's
+  "configure" and "make" process, by default, records some
+  system-specific information such as the hostname where Open MPI was
+  built and the username who built it.  If you desire a Reproducible
+  Build, set the $USER and $HOSTNAME environment variables before
+  invoking "configure" and "make", and Open MPI will use those values
+  instead of invoking "whoami" and/or "hostname", respectively.
+
 Platform Notes
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