Add location of readthedocs documentation to the README

[edwardhartnett]

@GeorgeGayno-NOAA I believe there is some official readthedocs documentation for UFS_UTILS. Can you put a link to it in this issue? Then I will add that link to the README, and it will help us getting this code documented and tested.

[GeorgeGayno-NOAA]

The readthedocs documentation is under ./docs/source It only includes information about chgres_cube currently.

[edwardhartnett]

Is the readthedocs documentation hosted on-line anywhere?

[GeorgeGayno-NOAA]

Is the readthedocs documentation hosted on-line anywhere?

https://noaa-emcufs-utils.readthedocs.io/en/latest/
https://readthedocs.org/projects/noaa-emcufs-utils/