add comment about read only tileset and zoom  

From http://osgeo-org.1560.x6.nabble.com/MapCache-returning-blank-tiles-when-outside-restricted-extents-td4373381.html  for the read only tileset
and from the https://github.com/mapserver/mapcache/blob/master/mapcache.xml.sample for the min max zoom restriction
Merge pull request #96 from stecyr/patch-2

add comment about read only tileset and zoom