add in-depth explanation of what the library does

[lukemuehring]

I'm a newcomer to this library (planning on using it to code something cool!), so I thought I would add to the README.md an in-depth explanation on how scores, grade scales, and conversion groups work.

[github-actions]

Coverage report

Caution

Coverage does not meet threshold
Statements coverage not met for global: expected <=999999 not covered statements, but got 630

St.❔	Category	Percentage	Covered / Total
🟢	Statements	87.87%	630/717
🟡	Branches	67.54%	129/191
🟢	Functions	81.25%	130/160
🟢	Lines	86.91%	571/657

Test suite run success

268 tests passing in 17 suites.

Report generated by 🧪jest coverage report action from 0dab2a8

[CocoisBuggy]

Thanks for your contribution!

I'm not opposed to expanding the README a little bit but reiterating implementation details is probably a little more verbose than is needed. It should probably be a single paragraph to communicate how grades within a conversion group map to each other.

There's also a feel of LLM-authored description here and it's a touch more editorialized than is strictly called for in a README