Add templates customization to main documentation

[sabueso]

HI!

Puput is great, and work out of the box! but will be great for newcomers (as me), show how to customize the default css&html templates.
Result a little bit tricky to find how to do it, and in fact, if you don't browse the closed issues you can't find how to do that (or maybe, I'm a little bit lazzy and i didn't find it in docs...), and it's really simple, as described here => #63

Maybe adding those instructions here? => http://puput.readthedocs.io/en/latest/setup.html

Thanks for these great and simple blog!

[sabueso]

WOA! It's duplicated #110 :-). Closed right now.