feat(write_file): add subject for the new exercise

subjects/devops/write_file/README.md

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+# Write file
+
+## Instructions
+
+Create a file `write_file.py` which will have a function `to_do(input)` with one input a `list[tuple[dt.date, str]]` which will be a to do list example:
+
+```python
+[
+    (dt.date(2022, 6, 1), "fix the seat"),
+    (dt.date(2022, 6, 2), "take the trash"),
+]
+```
+
+Your function needs to format and write the input in a text file `output.txt` as follow:
+
+```bash
+$ cat output.txt
+Wednesday 01 June 2022: fix the seat
+Thursday 02 June 2022: take the trash
+$
+```
+
+Make sure you follow the correct format for the output.
+
+## Usage
+
+Here is a possible `test.py` to test your function:
+
+```python
+import datetime as dt
+from write_file import to_do
+
+if __name__ == '__main__':
+    to_do_list = [
+        (dt.date(2022, 6, 1), "fix the seat"),
+        (dt.date(2022, 6, 2), "take the trash"),
+    ]
+    to_do(to_do_list)
+```
+
+```bash
+$ python test.py
+$ cat output.txt
+Wednesday 01 June 2022: fix the seat
+Thursday 02 June 2022: take the trash
+```
+
+## Hints
+
+- The `strftime() `method is used to format the date object as a string. You can use this method to format the date in the desired format (e.g. %A %d %B %Y).
+
+```python
+import datetime as dt
+
+date = dt.date(2022, 6, 1)
+formatted_date = date.strftime("%A %d %B %Y")
+print(formatted_date)
+```
+
+The output:
+
+```bash
+Wednesday 01 June 2022
+```
+
+- The `%A` and `%B` format codes are used to represent the full weekday and month name respectively. The `%d` format code is used to represent the day of the month. The `%Y` format code is used to represent the year.
+
+- The `write()` method is used to write a string to a file. You can use this method to write the formatted date and task to the output file.
+
+```py
+with open("output.txt", "w") as file:
+    file.write("Hello World!")
+```
+
+In this example, the `open()` function is used to open the file named `output.txt` with the write mode `w`. The `with` statement is used to open the file, and automatically close it after the indented block of code is executed.
+
+The `write()` method is used to write the string "Hello World!" to the file.
+
+- Make sure to include a newline character `\n` at the end of each line of output, to separate the tasks in the output file.
+
+- Test your function with different input formats, different date and task length, different date range and make sure that it works as expected.
+
+## References
+
+- [open a file](https://www.w3schools.com/python/python_file_write.asp)
+- [write in a file](https://www.pythontutorial.net/python-basics/python-write-text-file/)