docs(clean_the_list): add subject description

subjects/devops/clean_the_list/README.md

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+## clean_the_list
+
+### Instructions
+
+Create a file `shopping.py` that contains a function `clean_list` which takes in a list of strings and performs the following operations on each list item:
+
+- Removes all spaces before and after (but not between words).
+- Capitalizes the first letter (first letter only, other ones should be to lowercase).
+- Adds its index number before a separator `x/ `.
+- An empty list as input should return an empty list as output.
+- And don't forget the milk !!!! (add it at the end of the list if missing).
+
+### Usage
+
+Here is an example of how to use the clean_list function:
+
+```python
+import shopping
+shopping_list = ['tomatoes', 'pastas', 'milk', 'salt']
+print(shopping.clean_list(shopping_list))
+```
+
+This will output:
+
+```console
+['1x/ Tomatoes', '2x/ Pastas', '3x/ Milk', '4x/ Salt']
+```
+
+### References
+
+[string strip](https://www.w3schools.com/python/ref_string_strip.asp)
+[string capitalize](https://www.w3schools.com/python/ref_string_strip.asp)