Adds more documentation

For me bangsh balances simplicity and power in a great way. I wanted to learn how to use if and took some notes that I converted here to some more extensive documentation. Some things are pretty obvious, but the idea if to allow people to clearly see what bangsh can do for them. Let me know what you think.

added @bellthoven feedback
Merge pull request #15 from davidpelaez/master

Improve README file