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.

README.md

@@ -1,2 +1,55 @@
 # 017-followers
 
+Day 17 of the Udemy *100 Days of Code* course: a small example of how to
+define and use a Python class.
+
+This is a demonstration only. It has no plan, no tests and no dependencies.
+
+## What it shows
+
+The `User` class in [src/main.py](src/main.py) demonstrates:
+
+- a constructor (`__init__`) that sets attributes with default values
+- instance attributes (`id`, `username`, `followers`, `following`)
+- a method (`follow`) that changes the state of two objects at once
+
+```python
+class User:
+
+    def __init__(self, user_id, username):
+        self.id = user_id
+        self.username = username
+        self.followers = 0
+        self.following = 0
+
+    def follow(self, user):
+        user.followers += 1
+        self.following += 1
+```
+
+When `user_1.follow(user_2)` runs, `user_1.following` goes up by one and
+`user_2.followers` goes up by one.
+
+## Run it
+
+```bash
+python src/main.py
+```
+
+Expected output:
+
+```text
+0
+1
+1
+0
+```
+
+The four lines are `user_1.followers`, `user_1.following`,
+`user_2.followers` and `user_2.following`.
+
+## Layout
+
+| Path | Purpose |
+| --- | --- |
+| `src/main.py` | The `User` class and the demo that uses it |