docs: normalise Licence/License spelling across README

[millerjp]

Summary

British English is used throughout ("Licence", "behaviour", "artefact"). README badge alt-text uses License (American). The shield URL is out of our control but the alt-text and adjacent body text should be consistent British English.

Scope

Update README badge alt-text and any remaining "License" → "Licence" in prose. Keep the shield URL (shields.io) as-is.

Acceptance criteria

1. No License in body prose (only inside shield URLs).
2. Consistent use of "Licence" in headings.

Source: docs-writer.