Update submodule documentation to only fetch needed files

Author: Jules-Bertholet

.gitmodules

@@ -1,3 +1,5 @@
 [submodule "ckeditor5"]
 	path = ckeditor5
 	url = https://github.com/ckeditor/ckeditor5
+	update = checkout
+	shallow = true

README.md

@@ -233,5 +233,17 @@ To enter a development loop with hot reload support:
 
 ### Tests
 
-Use `yarn test`, optionally with `--coverage`, `--watch`, or `--source-map`, to run tests.
+Run the below commands from the repo root directory to fetch files needed to run tests (you will only need to do this once):
+
+```
+git submodule deinit ckeditor5
+rm -rf ckeditor5
+git clone --no-checkout --depth=1 --filter=blob:none https://github.com/ckeditor/ckeditor5
+rm -rf .git/modules/ckeditor5
+git submodule absorbgitdirs
+git -C ckeditor5 sparse-checkout set 'packages/ckeditor5-core/tests/_utils/**'
+git submodule update --init --force --checkout ckeditor5
+```
+
+Then, use `yarn test`, optionally with `--coverage`, `--watch`, or `--source-map`, to run tests.
 You will need to have run `git submodule init` and `git submodule update` first.