Describe external xrefs

Add rules to contrib guide for representing external links in rST.

Change-Id: I5b8e7fbb4ed35ec4da8a9d14acd64385ae021548
Signed-off-by: Ron Stone <ronald.stone@windriver.com>

doc/source/contributor/doc_contribute_guide.rst

@@ -1486,10 +1486,32 @@ Use the :command:`system help` command for the full list of options.
 Cross-references
 ****************
 
-Cross-reference to arbitrary locations in a document using the ``ref`` role and a
+.. begin-xrefs
-named target. Named targets must precede a section heading. For more information
+
-on references, see
+Cross-reference to arbitrary locations in a document using the ``ref`` role and
-`Internal Hyperlink Targets <http://docutils.sourceforge.net/docs/user/rst/quickref.html#internal-hyperlink-targets>`_.
+a named target. Named targets must precede a section heading. For more
+information on references, see `Internal Hyperlink Targets
+<http://docutils.sourceforge.net/docs/user/rst/quickref.html#internal-hyperlink-targets>`_.
+
+When using external references, be sure to prefix the URL with either
+``http://`` or ``https://``. For example:
+
+.. code-block::
+
+   `Internal Hyperlink Targets
+   <http://docutils.sourceforge.net/docs/user/rst/quickref.html#internal-hyperlink-targets>`_.
+
+.. note::
+
+   When providing an example of a URL format, treat the example as code by
+   either placing it in a ``.. code-block::`` or in double backticks, for
+   example:
+   
+   .. code-block::
+
+      ``https://<fqdn>/<path>``
+
+.. end-xrefs
 
 **Input:**
 
@@ -1590,8 +1612,8 @@ Open the ``/path/to/file.name`` file.
 
 Optionally pass the ``--option`` with the command.
 
-Refer to the
+Refer to the `OpenStack Inline elements guide
-`OpenStack Inline elements guide <https://docs.openstack.org/doc-contrib-guide/rst-conv/inline-markups.html>`_
+<https://docs.openstack.org/doc-contrib-guide/rst-conv/inline-markups.html>`_
 for markup for other inline elements.
 
 *****