docs: add deprecation notice pointing to aw-notify-rs

[0xbrayo]

Fixes #14 by adding a clear notice at the top of the README that this Python implementation is being replaced by the Rust rewrite (aw-notify-rs). This helps users understand they should use the newer Rust version instead.

[greptile-apps]

Greptile Summary

This documentation-only PR adds a prominent README notice directing new users from the Python implementation to the official Rust rewrite.

• Links directly to ActivityWatch/aw-notify-rs.
• Clarifies which implementation new users should choose.

Confidence Score: 5/5

The PR appears safe to merge.

The new notice accurately identifies and links the official Rust replacement, and no blocking or independently actionable issue remains.

Important Files Changed

Filename	Overview
README.md	Adds an accurate, visible deprecation notice and replacement link without introducing actionable documentation issues.

_{Reviews (1): Last reviewed commit: "docs: add deprecation notice pointing to..." | Re-trigger Greptile}