Update README.md

Author: justinkames

README.md

@@ -68,15 +68,20 @@ new Vue({
             b : 'b'
         }
     },
-
     created() {
         this.$log.debug('test', this.a, 123)
         this.$log.info('test', this.b)
         this.$log.warn('test')
         this.$log.error('test')
         this.$log.fatal('test')
+        externalFunction()
     }
 })
+
+function externalFunction() {
+   // log from external function
+   Vue.$log.debug('log from non Vue function.') 
+}
 ```
 
 ## Maintainers