DOC: update io.rst
![image](https://cloud.githubusercontent.com/assets/953992/24471918/c222f848-1491-11e7-8d17-d76da3204399.png)

redo of the top of the io.rst page. was getting a bit long / disorganized. I added links for the various formats as well. *could* add another column about use (e.g. general, portable, flexible, etc), but maybe getting much
