DOC: Update the doc about Sql IO

[garaud]

I don't know if the piece of code to read/write into SQL database such as Sqlite is robust enough to write something about this (great) feature in the doc (i.e "io.rst" file).

I would like to write a tiny section about 'execute', 'tquery' & 'read_frame' methods with some examples.
Do you think that it's relevant?

Cheers,
Damien

[wesm]

Go for it