#144 More details on recurring contributions

Created by: mikaweinstein

I added more detail to how you configure recurring payments on the Contribution Page, as well as fleshing out the Support for recurring contributions section. It currently does not cover generally configuring payments with an IPN (which there is some documentation for on the wiki page). Should this be in the user guide or developer guide? Either way, I'll leave that documentation update to someone else, as I'm not super familiar with the process.

I also included instructions to get to the iATS admin page--I don't think that that's really the place for it (since it doesn't only relate to recurring) but there doesn't seem to be a better place to put it while payment processor details are still linked to the wiki. And, it's a nice feature that should be pointed out somewhere.