create logger in DemoPluginFactory

demo/app/src/main/java/org/pf4j/demo/DemoPluginFactory.java

@@ -15,15 +15,18 @@
  */
 package org.pf4j.demo;
 
+import java.lang.reflect.Constructor;
 import org.pf4j.DefaultPluginFactory;
 import org.pf4j.Plugin;
 import org.pf4j.PluginWrapper;
 import org.pf4j.demo.api.PluginContext;
-
-import java.lang.reflect.Constructor;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 class DemoPluginFactory extends DefaultPluginFactory {
 
+    private static final Logger log = LoggerFactory.getLogger(DemoPluginFactory.class);
+
     @Override
     protected Plugin createInstance(Class<?> pluginClass, PluginWrapper pluginWrapper) {
         PluginContext context = new PluginContext(pluginWrapper.getRuntimeMode());