[Docs]: Update README to reflect current addon state

[Xerrion]

Problem

The README is out of date relative to the addon's current state on master:

• The new /phd config slash command and AceConfig options panel (PR feat: add AceConfig options UI for overlay positioning (#40) #41, commit 690d9ce) are not documented.
• The Features list does not mention the configurable action-bar overlay (anchor, offset, font size, abbreviate-numbers).
• The slash-command table is missing /phd config.
• The 4-layer architecture (Core / Engine / Data / Presentation) used throughout the codebase is not introduced.
• The 36-file busted test suite is not mentioned for contributors.

Proposed Solution

Refresh README.md so it accurately reflects the addon as it sits on master. Add a Configuration section, add /phd config to the commands table, briefly introduce the architecture, and note luacheck and busted for contributors. No source code changes.

Alternatives

Wait for release-please to cut the version containing PR #41 and document then. Rejected: master is the source of truth and the README should match it.