docs: Added TypeScript info to the Installing Slate walkthrough

docs/walkthroughs/01-installing-slate.md

@@ -47,6 +47,25 @@ const App = () => {
 
 Of course we haven't rendered anything, so you won't see any changes.
 
+> If you are using TypeScript, you will also need to extend the `Editor` with `ReactEditor` as per the documentation on [TypeScript](/concepts/11-typescript). The example below also includes the custom types required for the rest of this example.
+
+```ts
+// TypeScript Users only add this code
+import { BaseEditor } from 'slate'
+import { ReactEditor } from 'slate-react'
+
+type CustomElement = { type: 'paragraph'; children: CustomText[] }
+type CustomText = { text: string }
+
+declare module 'slate' {
+  interface CustomTypes {
+    Editor: BaseEditor & ReactEditor & HistoryEditor
+    Element: CustomElement
+    Text: CustomText
+  }
+}
+```
+
 Next we want to create state for `value`:
 
 ```jsx