docs: update readme to include model alternative

[stephantul]

Simple readme update: we didn't mention the possibility of using a custom model anywhere.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[greptile-apps]

Confidence Score: 5/5

The documentation-only change appears safe to merge.

The documented environment variable is passed unchanged to the Model2Vec loader, and no concrete inaccurate instruction or reachable failure was established.

_{Reviews (1): Last reviewed commit: "docs: update readme to include model alt..." | Re-trigger Greptile}