Improve documentation

We should improve the documentation for this module overall. The README.txt seems to be the most complete for the moment, and I've added a link to the raw file in the Project node. That said, it would be good to have:

* the main project page expose more details about what the module can do (in conjunction with an official and recommended 2.x release)
* a proper Documentation Guide in the d.o handbook pages for the module, that could perhaps elaborate a bit further on the README.txt w/rt advanced features like recurring/batch transactions, etc.