Support /etc/ostree/remotes.d

For many OS install scenarios, one runs through an installer which may
come with embedded data, and then the OS is configured post-install to
receive updates.

In this model, it'd be nice to avoid the post-install having to rewrite
the /ostree/repo/config file.

Additionally, it feels weird for admins to interact with "/ostree" -
let's make the system feel more like Unix and have our important
configuration in /etc.

https://bugzilla.gnome.org/show_bug.cgi?id=729343

doc/ostree.repo-config.xml

@@ -131,6 +131,19 @@ Boston, MA 02111-1307, USA.
     </variablelist>
 
   </refsect1>
+
+  <refsect1>
+    <title>/etc/ostree/remotes.d</title>
+
+    <para>
+      In addition to the <filename>/ostree/repo/config</filename>
+      file, remotes may also be specified in
+      <filename>/etc/ostree/remotes.d</filename>.  The remote
+      configuration file must end in <literal>.conf</literal>; files
+      whose name does not end in <literal>.conf</literal> will be
+      ignored.
+    </para>
+  </refsect1>
   
   <refsect1>
     <title>See Also</title>