Clear documentation on blacksmith requirements can reduce invalid bugs along the road

I have patched your documentation changing slightly the sections so it is clear that it is important to have a directory structure before executing "blacksmith /path/to/site" I have noticed about 3 bugs that I feel may have existed because of the docs problem: 

https://github.com/flatiron/blacksmith/issues/83
https://github.com/flatiron/blacksmith/issues/79
https://github.com/flatiron/blacksmith/issues/69