Added build screenshot to README.md.

README.md

@@ -8,6 +8,8 @@ CKEditor 5 document editor build
 
 The document editor build for CKEditor 5, featuring the decoupled UI editor implementation. Read more about the [document editor build](https://ckeditor.com/docs/ckeditor5/latest/builds/guides/overview.html#document-editor) and see the [demo](https://ckeditor.com/docs/ckeditor5/latest/examples/builds/document-editor.html).
 
+![CKEditor 5 decoupled document editor build screenshot](https://c.cksource.com/a/1/img/npm/ckeditor5-build-decoupled-document.png)
+
 ## Documentation
 
 See: