docs: add installation and maintenance guide

tests/test_addon_metadata.py

@@ -50,6 +50,27 @@ def test_repository_hostname_derivation() -> None:
     assert repository_id == "0031621f"
     assert f"{repository_id}-stelloauth" == "0031621f-stelloauth"
 
+
+def test_documentation_contract() -> None:
+    documentation = "\n".join(
+        (ROOT / path).read_text(encoding="utf-8")
+        for path in ("README.md", "stelloauth/README.md", "stelloauth/DOCS.md")
+    )
+    for required_text in (
+        REPOSITORY_URL,
+        "http://0031621f-stelloauth:8080/worker",
+        "http://192.168.1.20:8080/worker",
+        "Brand: Opel",
+        "Country: DK",
+        "v0.6.0",
+        "0.5.10",
+        "CloakBrowser Binary License",
+        "approximately 4 GB free",
+        "Login-flow RAM: not measured without real MyOpel credentials.",
+    ):
+        assert required_text in documentation
+    assert "deaktivér porttilknytningen igen" in documentation
+
 def test_translations_cover_every_option() -> None:
     keys = set(load_yaml("stelloauth/config.yaml")["options"])
     for language in ("da", "en"):