Writing doc about I/O SQL features. See #2541
Hi there, 

I wrote some contents about I/O SQL pandas features:
- some examples about the `read_frame` function with SQLite ;
- list other functions such as `tquery` (without example) ;
- write a tiny note about `write_frame` function.

I'm not sure about the title of this section. Any proposal is welcome ! Don't hesitate to comment the content (typo, misunderstanding, ...)
## Cheers

Damien G.

closes #2541 
 thanks!
