Add a brief ointer to the HACKING file

Signed-off-by: Ralph Castain <rhc@open-mpi.org>

README

@@ -15,7 +15,7 @@ Copyright (c) 2007      Myricom, Inc.  All rights reserved.
 Copyright (c) 2008-2017 IBM Corporation.  All rights reserved.
 Copyright (c) 2010      Oak Ridge National Labs.  All rights reserved.
 Copyright (c) 2011      University of Houston. All rights reserved.
-Copyright (c) 2013-2015 Intel, Inc. All rights reserved
+Copyright (c) 2013-2017 Intel, Inc. All rights reserved.
 Copyright (c) 2015      NVIDIA Corporation.  All rights reserved.
 Copyright (c) 2017      Los Alamos National Security, LLC.  All rights
                         reserved.
@@ -719,6 +719,11 @@ Open MPI Extensions
 Building Open MPI
 -----------------
 
+If you have checked out a DEVELOPER'S COPY of Open MPI (i.e., you
+cloned from Git), you really need to read the HACKING file before
+attempting to build Open MPI. Really.
+
+If you have downloaded a tarball, then things are much simpler.
 Open MPI uses a traditional configure script paired with "make" to
 build.  Typical installs can be of the pattern: