Document required submodule initialization

README.md

@@ -8,10 +8,23 @@ The status page is deployed to GitHub pages and will be built on each push to th
 
 ## Local development
 
+__Important__: The `cstate` theme is distributed as a git submodule contained in this repository, and is not pulled by default.
+To obtain the theme, you can either use `--recursive` when cloning:
+
+```console
+git clone --recursive [email protected]:wbstack/status-page.git
+```
+
+or init submodules from within the repository root
+
+```console
+git submodule update --init
+```
+
 To develop the site locally, use the existing [`docker compose`](https://docs.docker.com/compose/) setup.
 Running
 
-```
+```console
 docker compose up
 ```