Add brief overview to readme

[stephenworsley]

Add enough documentation to the readme so that new users should be able to make use of the most important features by following the example.

[codecov[bot]]

Codecov Report

Merging #152 (fd69b34) into unstructured_scheme (4f3a737) will not change coverage. The diff coverage is n/a.

@@                 Coverage Diff                  @@
##           unstructured_scheme     #152   +/-   ##
====================================================
  Coverage                99.56%   99.56%           
====================================================
  Files                       27       27           
  Lines                     2073     2073           
====================================================
  Hits                      2064     2064           
  Misses                       9        9           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 4f3a737...fd69b34. Read the comment docs.

[trexfeathers]

Already looking loads better IMO 🙂

[zklaus]

I think the main path to regridding should still follow the standard approach in Iris for regridding as documented here, don't you think?

As such, the first and principal example should be something along the lines of

result = source_cube.regrid(target_cube, esmf_regrid.ESMFRegrid())

[stephenworsley]

@zklaus Ultimately, I think the documentation probably would benefit from looking more like the iris documentation. With that said, the current implementation of regridders does not go so far as to offer an exact match to iris functionality (that is to say, the unstructured regridders do not currently support being called in this way). This documentation reflects that and describes the regridding functionality which is common between the structured and unstructured regridders. The cached regridder is also necessary to introduce to discus regridder saving, which is a new feature not contained in iris. The unstructured and structured regridders are currently a little disjointed in their functionality and I hope to be unify them in a future release, however now that UGRID support in iris has released, I'm keen to release what we have for unstructured regridding as soon as possible, even if the structured and unstructured regridders do not yet have the same level of functionality.