include an example

Andreas Eichner · rouault · commit 34bb73cf519d · 2017-03-07T16:37:46.000+01:00
diff --git a/mapcache.xml.sample b/mapcache.xml.sample
@@ -559,6 +559,7 @@
       <quality>75</quality>  
 
       <photometric>RGB</photometric>   <!-- RGB | YCBCR -->
+      <optimize>true</optimize>  <!-- true | false | arithmetic -->
    </format>
    <format name="PNG_BEST" type ="PNG">
       <compression>best</compression>
