feat(days-and-numbers): add subject for the new exercise

subjects/devops/days_and_numbers/README.md

@@ -0,0 +1,99 @@
+## Days and numbers
+
+### Instructions
+
+Create a file `custom_calendar.py` which will have 2 functions:
+
+- `day_from_number(day_number)`
+- `day_to_number(day)`
+
+Those functions perform conversion between day `index` and day `word` and vice versa:
+
+- 1 = Monday
+- 2 = Tuesday
+- 3 = Wednesday
+- 4 = Thursday
+- 5 = Friday
+- 6 = Saturday
+- 7 = Sunday
+
+You should return `None` if the input is invalid (invalid number or day string).
+
+### Usage
+
+Here is a possible `test.py` to test your functions:
+
+```python
+import custom_calendar
+
+if __name__ == '__main__':
+    print(custom_calendar.day_from_number(2))
+    print(custom_calendar.day_from_number(1))
+    print(custom_calendar.day_from_number(1000))
+    print(custom_calendar.day_to_number('Sunday'))
+    print(custom_calendar.day_to_number('invalid day'))
+```
+
+```console
+$ python test.py
+Tuesday
+Monday
+None
+7
+None
+```
+
+### Hints
+
+Dictionaries:
+
+A dictionary is a data type similar to arrays, but works with keys and values instead of indexes. Each value stored in a dictionary can be accessed using a key, which is any type of object (a string, a number, a list, etc.) instead of using its index to address it.
+
+For example, a database of phone numbers could be stored using a dictionary like this:
+
+```python
+phonebook = {}
+phonebook["John"] = 938477566
+phonebook["Jack"] = 938377264
+phonebook["Jill"] = 947662781
+print(phonebook)
+```
+
+output:
+
+```console
+{'John': 938477566, 'Jack': 938377264, 'Jill': 947662781}
+```
+
+Alternatively, a dictionary can be initialized with the same values in the following notation:
+
+```python
+phonebook = {
+    "John" : 938477566,
+    "Jack" : 938377264,
+    "Jill" : 947662781
+}
+print(phonebook)
+```
+
+Dictionaries can be iterated over, just like a list. However, a dictionary, unlike a list, does not keep the order of the values stored in it. To iterate over key value pairs, use the following syntax:
+
+```python
+phonebook = {"John": 938477566, "Jack": 938377264, "Jill": 947662781}
+for name, number in phonebook.items():
+    print("Phone number of %s is %d" % (name, number))
+```
+
+output:
+
+```console
+Phone number of John is 938477566
+Phone number of Jack is 938377264
+Phone number of Jill is 947662781
+```
+
+### Notions
+
+- [None type](https://www.w3schools.com/python/ref_keyword_none.asp)
+- [Dictionaries](https://docs.python.org/3/tutorial/datastructures.html#dictionaries)
+- [Access an item in dictionary](https://www.w3schools.com/python/python_dictionaries_access.asp)