Add HAMT Documentation to Repo

[Faolain]

Motivation:
With the addition of the HAMT for storing/retrieving metadata within ipldstore it would be worthwhile to add some additional documentation to the README to explain how the HAMT works (short one line or two examples of how data is being stored within the metadata). For example to @TheGreatAlgo's point of

Is the key here the position in the zarr for which we are requesting data? i.e "24.35.23"

with an example list of how the data is stored. E.g.

"24.35.23": bayfre.......,
"24.35.24": bayfre.......,

Additionally some additional comments in the code likely would be helpful for users looking to contribute or improve the codebase (especially as this code can hopefully be pushed upstream) especially describing what's happening in the ipldstore/arguments getting passed and so on.

Acceptance Criteria:

• Add documentation to README explaining the HAMT, keys, and how it's used
• Document arguments with specificity /examples within comments inside the ipldstore.py and the hamtwrapper