Document bash completion.

deps/rabbitmq_management/priv/www-cli/index.html

@@ -50,7 +50,15 @@
           import and export configuration.
         </li>
       </ul>
-   </p>
+    </p>
+
+    <p>
+      <code>rabbitmqadmin --bash-completion</code> will print a bash
+      completion script for rabbitmqadmin. This should be redirected
+      to a file and <code>source</code>d. On Debian-derived systems,
+      copy the file to <code>/etc/bash_completion.d</code> to make it
+      available system-wide.
+    </p>
 
     <h2>Examples</h2>