docs(libanki): README.md

[david-allison]

Note

Assisted-by: Claude Opus 4.8 - initial draft, heavily modified

Preview: https://github.com/david-allison/Anki-Android/blob/libanki-readme/libanki/README.md

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[lukstbit]

LGTM, left some notes on wording.