Add User class followers demo and README

Plan waived by the user in chat for this request: a demo of a Python class, no planning or tests.

src/main.py

@@ -0,0 +1,32 @@
+"""Demo of a simple `User` class with follower counts."""
+
+
+class User:
+    """A user that can follow other users."""
+
+    def __init__(self, user_id: str, username: str) -> None:
+        self.id = user_id
+        self.username = username
+        self.followers = 0
+        self.following = 0
+
+    def follow(self, user: "User") -> None:
+        """Follow another user, updating both users' counters."""
+        user.followers += 1
+        self.following += 1
+
+
+def main() -> None:
+    user_1 = User(user_id="001", username="Tirsvad")
+    user_2 = User(user_id="002", username="Tirsvad")
+
+    user_1.follow(user_2)
+
+    print(user_1.followers)
+    print(user_1.following)
+    print(user_2.followers)
+    print(user_2.following)
+
+
+if __name__ == "__main__":
+    main()