RFR: 7903859: add more details to the top-level README.md file

[Iris Clark]

On Wed, 2 Oct 2024 22:21:57 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> Please review a docs update, mostly correcting or adding details to the top-level README.md, but also taking this opportunity to add some details into the "man" page.

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/apidiff/pull/24#pullrequestreview-2344362392