Add section with basic service explanation

README.md

@@ -24,3 +24,31 @@ Expect often updates as we discuss & decide upon different things.
 
 * If the model property spans multiple relations, it should better be a selector.
 * If a model property, added to some list API, will cause `N + 1` problem that cannot be easily solved with `select_related`, it should better be a selector.
+
+## Services
+
+A service is a simple function that:
+
+* Takes keyword-only arguments
+* Is type-annotated (even if you are not using `mypy` at the moment)
+* Does business logic - from simple model creation to complex cross-cutting concerns, to calling external services & tasks.
+
+An example service that creates an user:
+
+```python
+def create_user(
+    *,
+    email: str,
+    name: str
+) -> User:
+    user = User(email=email)
+    user.full_clean()
+    user.save()
+
+    create_profile(user=user, name=name)
+    send_confirmation_email(user=user)
+
+    return user
+```
+
+As you can see, this service calls 2 other services - `create_profile` and `send_confirmation_email`