add documentation about git submodules and nix

Author: juliankuners

README.md

@@ -7,3 +7,7 @@ To keep your project up to date with the most recent version of this template, w
 ```bash
 cruft create https://github.com/runtimeverification/python-project-template
 ```
+
+### Notes
+#### Git submodules
+If your new project contains git submodules that are not dependencies of any nix derivations, you should exclude the git submodule in the nix derivation in `nix/{{cookiecutter.project_slug}}/default.nix`. See [runtimeverification/k/pull/4804](https://github.com/runtimeverification/k/pull/4804) for more information.