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.