Bugfix and hybrid coordinate example from Pragallva + additional contribution + readme files

[csyhuang]

This PR includes contribution by Pragallva to fix #102 : the differential area to compute reference states, $a^2 \cos\phi \Delta\phi \Delta\lambda$ , was miscomputed as $a^2 \cos\phi (\Delta\phi)^2$ . (This has not caused issues in results presented in our previous publications because we have been using ERA5 datasets with $\Delta\phi = \Delta\lambda$.)

Summary of changes:

1. $\Delta\phi$ and $\Delta\lambda$ are precomputed in QGField object and passed into the F2PY modules as parameters
2. Pragallva has included an example in scripts/pre_process_data_in_hybrid_coordinates/ how to preprocess climate model output in hybrid coordinate for consumption of QGField downstream by (1) transforming the fields to pressure coordinates, and (2) do gridfill over underground" gridpoints using poisson solver (as how Clare did in MDTF project)
3. Added CONTRIBUTING.md to include instructions for potential contributors.

@chpolste please let me know if you have time to review in the coming 3 weeks. Thanks! 😊

[chpolste]

Looks good to me 👍
No problems when running tests and building documentation on my local machine either.

[csyhuang]

@chpolste Thanks Christopher! I'll go ahead merge it and do a bugfix release!