Add "Pin Component Format" to README.md

The README.md has the pin component format [as a TODO item](https://github.com/adafruit/Wippersnapper_Components#pin-component-format).

The format used by pin components is reflected by the [pin/schema.json file](https://github.com/adafruit/Wippersnapper_Components/blob/main/components/pin/schema.json) in this repository.

You may either (simple) link to the schema from within the README file, or (better) explain the schema in the README as a table/visually as you best see fit.

Start with the TODO item in README.md and inspect components/pin/schema.json, which defines the pin component format. Either link to the schema from the README or explain its fields in a table or visual summary. Done means the TODO is replaced with useful pin-format documentation.

Written by the indexing model from the issue text.