Add description of task 0

src/day_1/tasks.md

@@ -1,5 +1,47 @@
 # Tasks
 
+## Task 0: Collective-Score
+
+This course uses [collective-score](https://codeberg.org/mo8it/collective-score) for interactive tasks.
+I spent many days programming it for you 😇
+
+As an attendee of the course, you will find the binary `cs` on the Linux system provided to you.
+Check it by running `cs --help`.
+
+You should use it to validate that you have done a task correctly.
+But first, you need to register yourself to be able to communicate with the Collective-Score server and see your progress on the projector.
+
+Run the following command **after replacing `USERNAME` with your username** (you can choose whatever username you want to be displayed to others):
+
+```bash
+cs user register USERNAME
+```
+
+If you get any errors, ask the tutor before proceeding with the tasks!
+
+Now, run the following command:
+
+```bash
+echo "Test" | cs task collective-score-intro
+```
+
+The check that `cs` does should fail.
+This is a demonstration of how checking a task fails and how you (hopefully) get a hint.
+
+In general, you will do a task and then run `cs task TASKCODE` to validate your solution.
+`TASKCODE` is a code unique to every task/subtask.
+For this demo task, the task code is `collective-score-intro`.
+
+Now, for this demo task to pass the check, run the following command:
+
+```bash
+echo "OK" | cs task collective-score-intro
+```
+
+Congratulations, you have done your first task 🎉
+
+To see your progress at any time, run the command **`cs progress show`**.
+
 ## Task 1: Building houses
 
 In this task, you will build a house with different rooms using (nested) directories.