[Doc] Complete README with Quick Start, Support Matrix, and contributor navigation

[lcy-seso]

Problem

The README currently has three critical gaps that hurt both user adoption and contributor onboarding:

1. Quick Start is TODO

Line 82 of README.md is just TODO. A working code example already exists in the HTML comments but is not rendered. New users landing on the repo have no way to evaluate or try the library.

2. Benchmark Summary and Support Matrix are TODO

Lines 35 and 44 of README.md. A complete Support Matrix table already exists in the HTML comments but is hidden. Without these, the project lacks credibility — users cannot tell which operators are available or how they perform.

3. No contributor navigation

The Documents section only explains the 4-layer architecture concept. There are no links to docs/CONTRIBUTING.md or docs/DEVELOPMENT.md. New contributors have no way to discover these critical files from the README.

Proposed Changes

• Uncomment and update the Quick Start example (GQA decode or another representative operator)
• Uncomment and update the Support Matrix table
• Add a Benchmark Summary section (even a brief one linking to benchmarks/)
• Add a "Contributing" section to the README linking to docs/CONTRIBUTING.md and docs/DEVELOPMENT.md
• Uncomment the PyPI badge once the package is published