Instructions for wakari

This PR contains instructions for installing Iris and Cartopy in wakari by setting up a custom conda environment. It does not include instructions for enabling packed PP or adding the GRIB API. It does not get you to a point where all the tests pass (due in part to wakari using the latest scipy and numpy packages), but Iris and Cartopy can be imported, netcdf data can be loaded from an opendap server and plots can be created with data and coastlines.

Fixes Scitools/iris#777