Add example of how to set purge_type parameter

docs/drivers.md

@@ -26,10 +26,16 @@ services:
             - {name: ezplatform.http_cache.purge_client, purge_type: myhttpcache}
 ```
 
-Any service which implements the PurgeClientInterface must be tagged with `ezplatform.http_cache.purge_client` in order
-to be registered as such.
-`purge_type` specifies what the value of the purge_type setting in `app/config/ezplatform.yml` should be in order to
-enable this driver.
+Any service which implements the PurgeClientInterface must be tagged with `ezplatform.http_cache.purge_client` in
+order to be registered as such.
+
+`purge_type` specifies what the value of the `ezpublish.http_cache.purge_type` setting in `app/config/ezplatform.yml`
+should be in order to enable this driver. By default this is set using `%purge_type%` parameter, and can be set in `app/config/parameters.yml` like so:
+
+```
+parameters:
+    purge_type: myhttpcache
+```
 
 
 ## TagHandler