docs: comprehensive README with integration examples

[happy-edge]

Summary

Rewrote the README to be more helpful for integrators.

Added

• Badges: npm version, MIT license
• Feature highlights: Quick overview of capabilities
• Quick start guides: Both script tag and npm approaches
• Display mode examples: Modal, integrated, and widget with code
• Configuration docs: Form props, wallet passthrough, callbacks, styling
• Links: Playground, full docs, contributing guide
• Local dev instructions: How to run locally

Why

The previous README was minimal (just 9 lines). New integrators had to dig through code or external docs to understand how to use the plugin. This README serves as a complete getting-started guide.

Screenshot

The new README provides copy-paste ready code for all common use cases.

[vercel]

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[thejesh23]

Big improvement over the 9-line original. Two concrete issues worth fixing before merge:

1. Hardcoded plugin-1.0.13.js in the script-tag example will bit-rot at the next version bump (and the README is exactly where new integrators will copy from). Worth either pointing at a stable/latest URL, or adding a clear "replace 1.0.13 with the current published version" note with a link to npm/releases. PR #221 in this same repo is wrestling with the same bundle-name problem from the dev side.

2. initialAmount: '1000000' // in lamports is incorrect when the example pairs it with USDC as the input mint. USDC is 6 decimals; lamports is SOL's unit (9 decimals, 1 SOL = 10⁹ lamports). The right phrasing is "in the input mint's smallest/atomic unit" — otherwise integrators will under/overshoot amounts by 1000×.

Also worth sanity-checking that every prop name in the Form Props block (fixedMint, fixedAmount, swapMode values, etc.) matches the actual FormProps type — README examples that don't typecheck are a common drift point.