Added the regrid and interpolate section of the userguide.

I've put a build of this up at http://pelson.github.io/iris/html/userguide/interpolation_and_regridding.html

For significant editorial comments, please consider submitting a PR against this branch.
The area weighted total is *approximately* conserved by the looks of it (try printing the area weighted sum of the cubes in your example below).

Secondly this might be an opportunity to mention that this is an appropriate method to regrid high resolution data onto a lower resolution grid since it will account for all data values, whereas linear interpolation will sub-sample and not include some data points. This is a common task and something that I often see done incorrectly with linear interpolation.