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Add external reference to the advanced ACC tutorial

(cherry picked from commit 3f04700)

modules/acc/doc/acc_admin.xml

@@ -97,7 +97,12 @@
 	<para>
 		The SQL, Event Interface and AAA backend support are compiled in the
 		module.
-        </para>
+	</para>
+	<para>
+	A very comprehensive description of how the accounting module works in 
+	terms accounting scope, accounting events and accounting backends can 
+	be found in this online <ulink url="https://www.opensips.org/Documentation/Tutorials-Advanced-Accounting/">Advanced Accounting Tutorial</ulink>.
+	</para>
 	<section>
 		<title>General Example</title>
 		<programlisting format="linespecific">