docs (#151): add slots unification section to migration guide (#159)

* docs (#151): add slots unification section to migration guide

* fix: typo

Co-authored-by: Natalia Tepluhina <tarya.se@gmail.com>

Co-authored-by: Natalia Tepluhina <tarya.se@gmail.com>

Author: bencodezen

src/.vuepress/config.js

@@ -94,6 +94,7 @@ const sidebar = {
         'migration/functional-components',
         'migration/async-components',
         'migration/custom-directives',
+        'migration/slots-unification',
         'migration/keycode-modifiers'
       ]
     },

src/guide/migration/slots-unification.md

@@ -0,0 +1,61 @@
+# Slots Unification
+
+## Overview
+
+This change unifies normal and scoped slots in v3.
+
+Here is a quick summary of what has changed:
+
+- `this.$slots` now exposes slots as functions
+- **BREAKING**: `this.$scopedSlots` is removed
+
+For more information, read on!
+
+## Previous Syntax
+
+When using the render function, i.e., `h`, v2 used to define the `slot` data property on the content nodes.
+
+```js
+// 2.x Syntax
+h(LayoutComponent, [
+  h('div', { slot: 'header' }, this.header),
+  h('div', { slot: 'content' }, this.content)
+])
+```
+
+In addition, when referencing scoped slots, they could be referenced using the following syntax:
+
+```js
+// 2.x Syntax
+this.$scopedSlots.header
+```
+
+## Current Syntax
+
+In v3, render functions will have a `slots` option where they can be defined instead.
+
+```js
+// 3.x Syntax
+h(LayoutComponent, {
+  slots: {
+    header: () => h('div', this.header),
+    content: () => h('div', this.content)
+  }
+})
+```
+
+And when you need to reference scoped slots programmatically, they are now unified into the `$slots` option.
+
+```js
+// 2.x Syntax
+this.$scopedSlots.header
+
+// 3.x Syntax
+this.$slots.header
+```
+
+## Migration Strategy
+
+A majority of the change has already been shipped in 2.6. As a result, the migration can happen in one step:
+
+1. Replace all `this.$scopedSlots` occurrences with `this.$slots` in v3.