Add comment explaining where javelin logs are found

Document where javelin2 logs are kept to make it easier to
debug issues.

Change-Id: I3dc04e854a2166351958eb8b86f4645cc809460a

grenade.sh

@@ -228,6 +228,7 @@ function run_javelin() {
         cp $tempest_conf $javelin_conf
         # Make javelin write logs to javelin.log
         iniset $javelin_conf DEFAULT log_file $GRENADE_DIR/javelin.log
+        echo "Logs can be found at javelin.log"
     fi
 
     echo_summary "Running Javelin to $action resources"