Recursion Concepts in Python in Lesson 2

**Issue**
In the Function.py and functions.md file, there's alreasy a primer on creating and calling a function. But besides this, maybe a short refresher on recursion might help too

**Proposed Pull Request**
Added a short primer on recursion using the classic example of calculating factorials. Deliberately didn't include hadnling edge cases like floating point numbers and mentioned it as a sort of 'challenge' in the functions.md file