Documentation: Document optional RHEL7 repositories

On minimal install RHEL 7 servers (and perhaps other types of installs)
you need to enable a couple of optional repositories for the yum-builddep
utility to work correctly.  This patch documents those two optional
repositories.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

Documentation/intro/install/fedora.rst

@@ -63,6 +63,11 @@ The command below will create a temporary SPEC file::
       > /tmp/ovs.spec
 
 And to install specific dependencies, use the corresponding tool below.
+For some of the dependencies on RHEL you may need to add two additional
+repositories to help yum-builddep.
+
+subscription-manager repos --enable=rhel-7-server-extras-rpms
+subscription-manager repos --enable=rhel-7-server-optional-rpms
 
 DNF::