remote: Add "ostree remote summary" command

Downloads and prints a remote summary file and any signatures in an
easy-to-read format, or alternatively with the --raw option, prints
the summary GVariant data directly.

https://bugzilla.gnome.org/show_bug.cgi?id=759250

doc/ostree-remote.xml

@@ -66,6 +66,9 @@ Boston, MA 02111-1307, USA.
             <cmdsynopsis>
                 <command>ostree remote refs</command> <arg choice="req">NAME</arg>
             </cmdsynopsis>
+            <cmdsynopsis>
+                <command>ostree remote summary</command> <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">NAME</arg>
+            </cmdsynopsis>
     </refsynopsisdiv>
 
     <refsect1>
@@ -157,6 +160,20 @@ Boston, MA 02111-1307, USA.
         </variablelist>
     </refsect1>
 
+    <refsect1>
+        <title>'Summary' Options</title>
+
+        <variablelist>
+            <varlistentry>
+                <term><option>--raw</option></term>
+
+                <listitem><para>
+                    Show raw variant data
+                </para></listitem>
+            </varlistentry>
+        </variablelist>
+    </refsect1>
+
     <refsect1>
         <title>Example</title>
         <para><command>$ ostree remote show-url local</command></para>