Update the documentation to describe the overall Available Landsat imagery

[qjhart]

The README.md page needs a short description of the data that is available in this project and where it comes from.

We also need to describe our choice of projection and bounding box, so that people understand why we are using what we are using for this.

# with b as (select st_transform(boundary,26910) as b from bbox) select format('%s %s %s %s',(st_xmin(b)-120)::integer,(st_ymax(b)+120)::integer,(st_xmax(b)+120)::integer,(st_ymin(b)-120)::integer) as ul_lr from b;
ul_lr:=596898 4276385 656760 4162602