Add CONTRIBUTING.md

[jatinkrmalik]

Adds a short contributor guide. On-device after model download, Apple Silicon and macOS 14+, make test/build, PRs against main, Discord, AGPL-3.0. The README points at it.

Draft until CI is green.

[netlify]

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[greptile-apps]

Greptile Summary

Adds a contributor guide and links it from the README.

• Documents supported development hardware, macOS, and Xcode requirements.
• Clarifies test, build, run, install, pull-request, community, and licensing workflows.

Confidence Score: 5/5

The PR appears safe to merge.

The previously reported toolchain ambiguity is resolved by explicitly requiring Xcode for app builds and limiting standalone Swift support to some unit tests, so no blocking failure remains.

Important Files Changed

Filename	Overview
CONTRIBUTING.md	Adds contributor guidance and now explicitly clarifies that standalone Swift supports only some tests, while building the app requires Xcode.
README.md	Adds a direct link to the new contributor guide.

_{Reviews (2): Last reviewed commit: "Require Xcode in CONTRIBUTING setup" | Re-trigger Greptile}