Add events API change section ($on, $off and $once removal) (#167)

* feat: added events api section

* fix: renamed section to Migration Strategy

Author: NataliaTepluhina

src/.vuepress/config.js

@@ -94,6 +94,7 @@ const sidebar = {
         'migration/functional-components',
         'migration/async-components',
         'migration/custom-directives',
+        'migration/events-api',
         'migration/data-option',
         'migration/filters',
         'migration/fragments',

src/guide/migration/events-api.md

@@ -0,0 +1,64 @@
+---
+types:
+  - removed
+  - breaking
+---
+
+# `$on`, `$off` and `$once` methods removal <span v-for="type in $frontmatter.types" class="badge" :key="`type-${type}`">{{ type }}</span>
+
+## Overview
+
+`$on`, `$off` and `$once` instance methods are removed. Vue instances no longer implement the event emitter interface.
+
+## Previous Syntax
+
+In v2, Vue instance could be used to trigger handlers attached imperatively via the event emitter API (`$on`, `$off` and `$once`). This was used to create _event hubs_ to create global event listeners used across the whole application:
+
+```js
+// eventHub.js
+
+const eventHub = new Vue()
+
+export default eventHub
+```
+
+```js
+// ChildComponent.vue
+import eventHub from './eventHub'
+
+export default {
+  mounted() {
+    // adding eventHub listener
+    eventHub.$on('custom-event', () => {
+      console.log('Custom event triggered!')
+    })
+  },
+  beforeDestroy() {
+    // removing eventHub listener
+    eventHub.$off('custom-event')
+  }
+}
+```
+
+```js
+// ParentComponent.vue
+import eventHub from './eventHub'
+
+export default {
+  methods: {
+    callGlobalCustomEvent() {
+      eventHub.$emit('custom-event') // if ChildComponent is mounted, we will have a message in the console
+    }
+  }
+}
+```
+
+## 3.x Update
+
+We removed `$on`, `$off` and `$once` methods from Vue instance completely. `$emit` is still a part of the existing API as it's used to trigger event handlers declaratively attached by a parent component
+
+## Migration Strategy
+
+Existing event hubs can be replaced by using an external library implementing the event emitter interface, for example [mitt](https://github.com/developit/mitt).
+
+These methods can also be supported in compatibility builds.