Add an "about" file describing this repo's origins from the original ORNL DAAC notebook that inspired it

[chuckwondo]

Add an ABOUT.md or an ABOUT.ipynb file, describing the impetus for this project, specifically starting with the ORNL DAAC code in sections 4b and 4c of the 2_gedi_l4a_subsets.ipynb notebook.

To aid in comparing our code with the original notebook code, we should refer to the corresponding parts in our code, particularly the H5DataFrame class and the subset_hdf5 function (to avoid requiring the reader to sift through the code that's not relevant for comparison, such as our CLI code), and ideally this "about" file should include code snippets using our code (not necessarily via submitting a DPS) to achieve the same results (or very nearly so) as shown in the original notebook.

If an ABOUT.ipynb is created, that might be the best way to construct a "side-by-side" comparison between the code in this repo and the ORNL DAAC notebook.