docs: Add a "Downloading Open MPI" page

Sepcfically mention that automatically-generated GitHub.com tarballs
are not official releases, and will not work.

Signed-off-by: Jeff Squyres <jeff@squyres.com>

Author: jsquyres

docs/installing-open-mpi/downloading.rst

@@ -0,0 +1,41 @@
+
+.. _building-open-mpi-downloading-label:
+
+Downloading Open MPI
+====================
+
+Open MPI is generally available two ways:
+
+#. As source code.
+
+   * The best place to get an official Open MPI source code
+     distribution is from `the main Open MPI web site
+     <https://www.open-mpi.org/>`_.
+
+   * Downstream Open MPI packagers (e.g., Linux distributions)
+     sometimes also provide source code distributions.  They may
+     include additional patches or modifications.
+
+     Consult your favorite downstream packager for more details.
+     
+   .. caution:: Do **not** download an Open MPI source code tarball
+               from GitHub.com.  The tarballs automatically generated
+               by GitHub.com are incomplete and will not build
+               properly.
+
+               GitHub.com-generated tarballs are **not** official Open
+               MPI releases.
+
+#. As binary packages.
+
+   * The Open MPI community does not provide binary packages on `the
+     main Open MPI web site <https://www.open-mpi.org/>`_.
+
+   * Various downstream packagers (e.g., Linux distributions,
+     Homebrew, MacPorts, etc.) *do* provide pre-built, binary packages
+     for Open MPI.
+
+     Consult your favorite downstream packager for more details.
+
+The majority of the rest of this section deals with installing Open
+MPI from source.

docs/installing-open-mpi/index.rst

@@ -7,6 +7,7 @@ Building and installing Open MPI
    :maxdepth: 1
 
    quickstart
+   downloading
    supported-systems
    definitions
    filesystem-requirements

docs/installing-open-mpi/quickstart.rst

@@ -53,6 +53,14 @@ packages:
 Building from source
 --------------------
 
+Download the Open MPI source code from `the main Open MPI web site
+<https://www.open-mpi.org/>`_.
+
+.. caution:: Do **not** download an Open MPI source code tarball from
+             GitHub.com.  The tarballs automatically generated by
+             GitHub.com are incomplete and will not build properly.
+             They are **not** official Open MPI releases.
+
 Open MPI uses a traditional ``configure`` script paired with ``make``
 to build.  Typical installs can be of the pattern: