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.
Too many commas in one sentence I think. I suggest:
```
Interpolation can be achieved on a cube with the iris.cube.Cube.interpolate
method, with the first argument being the points to interpolate, and the second being
the interpolation scheme to use. The result is a new interpolated cube.
```