Document subsetting a subset output

[chuckwondo]

Users may produce a subset that includes a number of columns and a potentially large area of interest, but may not always need to read all columns over the entire AOI.

We should provide examples of the following when reading an output file produced by the subsetter:

1. selecting only necessary columns instead of all of them
2. selecting only data within a geometry that is smaller than the AOI represented by the entire file

[wildintellect]

This is directly connected to another MAAP objective on documenting cloud optimized access of vectors. cc: @smk0033