doc: pg_prewarm add configuration example

[Dong Wook Lee]

 Hi hackers,

 I thought it would be nice to have an configuration example of the pg_prewarm 
extension.
 Therefore, I have written an example of a basic configuration.

 ---
 Regards,
 DongWook Lee


diff --git a/doc/src/sgml/pgprewarm.sgml b/doc/src/sgml/pgprewarm.sgml
index 4e68812250..754a21a076 100644
--- a/doc/src/sgml/pgprewarm.sgml
+++ b/doc/src/sgml/pgprewarm.sgml
@@ -121,6 +121,19 @@ autoprewarm_dump_now() RETURNS int8
     </listitem>
    </varlistentry>
   </variablelist>
+  <para>
+   These parameters set in <filename>postgresql.conf</filename>.
+   Typical usage might be:
+  </para>
+
+<programlisting>
+# postgresql.conf
+shared_preload_libraries = 'pg_prewarm'
+
+pg_prewarm.autoprewarm = true
+pg_prewarm.autoprewarm_interval = 300s
+
+</programlisting>
 
  </sect2>