remote-add: Add a --collection-id argument to the built-in add command

This allows newly configured remotes to have their collection ID specified,
so that refs from them can be downloaded from peers as well as the
upstream collection, using the remote’s configuration.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #924
Approved by: cgwalters

man/ostree-remote.xml

@@ -137,6 +137,16 @@ Boston, MA 02111-1307, USA.
                   <command>ostree remote gpg-import --keyring=FILE</command>.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--collection-id</option>=COLLECTION-ID</term>
+
+                <listitem><para>
+                  Set the collection ID for the remote to a value provided by
+                  the repository owner, which allows refs from this remote to be
+                  shared peer to peer.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>