Add onfocus/onblur to textarea.

src/components/NoteList.vue

@@ -1,6 +1,8 @@
 <template>
   <textarea
     @keyup.enter="addNote"
+    onfocus="this.value=''"
+    onblur="this.value = 'Start typing something…'"
     v-model="content"
     cols="30"
     rows="10"