add warning for heavy load usage

src/main/webapp/templates/admin.settings.html

@@ -129,6 +129,7 @@
 						<label class="control-label" for="heavyLoad">Heavy load</label>
 						<div class="controls">
 							<input type="checkbox" name="heavyLoad" ng-model="settings.heavyLoad" />
+							<span class="help-block">Don't use this unless you know what you're doing!</span>
 						</div>
 					</div>
 					<div class="control-group">