Added code and description for this simple exercise: check social security number validity

Added a simple exercise. See .md file for more details.
Is it what a beginner in python would  write? 

I would first look for a built in function and if none exist build my own.

Personally I think it is better to learn and use built in functions rather then building your own, reinventing the wheel and all that. Although I see the point on not becoming a programmer dependant on the language you use.