add more to the upgrade guide

docs/src/docPages/overview/upgrade-guide.md

@@ -1,7 +1,8 @@
 # Upgrade Guide
 
-## Reasons to upgrade to tiptap 2.x
+The new API will look pretty familiar too you, but there are a ton of changes though. To make the upgrade a little bit easier, here is everything you need to know:
 
+## Reasons to upgrade to tiptap 2.x
 * TypeScript: auto complete, less bugs, generated API documentation
 * Amazing documentation with 100+ pages
 * Active maintenance, no more updates to 1.x
@@ -9,9 +10,27 @@
 * Less bugs, tested code based
 
 ## Upgrading from 1.x to 2.x
-The new API will look pretty familiar too you, but there are a ton of changes though. To make the upgrade a little bit easier, here is everything you should do:
 
-### New document type
+### 1. Explicitly register the Document, Text and Paragraph extensions
+
+Tiptap 1 tried to hide a few required extensions from you. Be sure to explicitly import the [Document](/api/extensions/document), [Paragraph](/api/extensions/paragraph) and [Text](/api/extensions/text) extensions.
+
+```js
+import Document from '@tiptap/extension-document'
+import Paragraph from '@tiptap/extension-paragraph'
+import Text from '@tiptap/extension-text'
+
+new Editor({
+  extensions: [
+      Document(),
+      Paragraph(),
+      Text(),
+      …
+  ]
+})
+```
+
+### 2. New document type
 **We renamed the default `Document` type from `doc` to `document`.** To keep it like that, use your own implementation of the `Document` node or migrate the stored JSON to use the new name.
 
 ```js
@@ -26,7 +45,7 @@ new Editor({
 })
 ```
 
-### New extension API
+### 3. New extension API
 
 In case you’ve built some custom extensions for your project, you’ll need to rewrite them to fit the new API. No worries, though, you can keep a lot of your work though. The schema, commands, keys, inputRules, pasteRules all work like they did before. It’s just different how you register them.
 
@@ -34,5 +53,5 @@ In case you’ve built some custom extensions for your project, you’ll need to
 const CustomExtension = …
 ```
 
-### Blockquotes must not be nested anymore
+### 4. Blockquotes must not be nested anymore