Update building-from-sources documentation to mention git submodules (p…

…yodide#5220)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/development/building-from-sources.md

@@ -24,6 +24,23 @@ Pyodide can be built from sources on different platforms,
 
 ## Build instructions
 
+The Pyodide repository has a git submodule called `pyodide-build`. Make sure to
+do a recursive clone:
+
+```bash
+git clone --recursive https://github.com/pyodide/pyodide
+```
+
+or if you have already cloned the Pyodide repository without the `--recursive`
+flag, you may initialize the submodule with:
+
+```bash
+git submodule update --init
+```
+
+If you change git branches, make sure you update `pyodide-build` with
+`git submodule update`.
+
 ### Using Docker
 
 We provide a Debian-based x86_64 Docker image