Update Canopy Cover enhancement documentation

[dlebauer]

Move details from https://docs.google.com/document/d/1LRAtjwzyaJ5KzBiJ-4yaIorvLL4-g31OUUQNveeVHz4/edit to the README in https://github.com/terraref/extractors-stereo-rgb/tree/master/canopy-cover-enhancement

Also add citation from Li et al CVPPP 2019