Add helper modules for bats

According to these instructions:
https://bats-core.readthedocs.io/en/stable/tutorial.html#quick-installation

README.md

@@ -13,6 +13,12 @@ You include a small bash script inside your commit messages, and `git-check-asse
 
 ⚠️ Only run this on repositories and branches that you trust, since the `bash` scripts in the commit messages can do whatever they want.
 
+## Building
+
+```bash
+nix build .?submodules=1
+```
+
 ## Examples of commit messages
 
 Assert that a commit builds: