docs: note about submodule, close antfu#61

CONTRIBUTING.md

@@ -1 +1,19 @@
 Please refer to https://github.com/antfu/contribute
+
+---
+
+## Additional Notes
+
+This repository contains a [git submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules) to `vscode-textmate`. By default `git clone` does not clone submodules. To clone this repository and its submodules, use:
+
+```bash
+git clone --recursive https://github.com/antfu/shikiji
+```
+
+Or if you have already cloned it, use:
+
+```bash
+git submodule update --init --recursive
+```
+
+Learn more at this [StackOverflow thread](https://stackoverflow.com/a/4438292).