Add better documentation about the templating system and the attribute parameters

[refnil]

@nmattia yes, it does (thank you by the way!), though I think the role of attributes should be clarified, at least on the readme, what do you think? The relationship between attributes and templates is never explained explicitly...

Originally posted by @sir4ur0n in #232 (comment)

[nmattia]

The --help says this:

 -t,--template URL        Used during 'update' when building URL. Occurrences
                           of <foo> are replaced with attribute 'foo'.

I guess something like this could be added to the README?