Improve README, documentation, and discoverability

[florianv]

What changed

README

• Rewrote around a 'plug-and-play Symfony' value proposition: drop-in bundle for currency conversion, configurable via config/packages/florianv_swap.yaml, container service ready to inject as Swap\Swap.
• New orientation sections (What is Symfony Swap?, When should you use Symfony Swap?, Why Symfony Swap and not raw Swap?, Common use cases, Which package should I use?), aligned with the rest of the ecosystem.
• Modernized the install path: PSR-18 first via symfony/http-client; the EOL php-http/guzzle6-adapter mention is gone.
• Modernized the registration path: register the bundle in config/bundles.php (Symfony 4+/5+/6+/7+/8+) instead of the pre-Symfony 4 AppKernel pattern.
• Replaced the APILayer-anchored quickstart with a neutral default (european_central_bank, free, no API key) and a fallback-chain example using priorities.
• Dropped the duplicated Services table that mirrored the Swap README. The README now points to the canonical Swap Providers table.
• Added cross-links to Swap, Exchanger, and Laravel Swap.
• Added a neutral Sponsorship section near the bottom.
• Added light section emojis for visual scanning.
• Dropped the legacy s3 logo image from the H1 for a cleaner header.

Resources/doc/index.md

• Dropped the legacy ## Sponsors block at the top and the 'we recommend the services that support our project' line in Configuration. No commercial provider is preferred.
• Added the same orientation sections at the top so the doc stands on its own and aligns vocabulary with the README.
• Modernized Installation (PSR-18 first, Guzzle 7 alternative noted; guzzle6-adapter removed).
• Modernized Setup to config/bundles.php and config/packages/.
• Added a Provider configuration table mapping each commercial identifier to its required option (api_key, access_key, app_id, token, api-key) and optional flags (enterprise, paid). Public providers and the array fixture are documented separately.
• Added a Cache configuration subsection covering the built-in array / apcu / filesystem types and PSR-16 service ID injection.
• Updated Usage to modern Symfony patterns (constructor injection with #[Autowire(service: 'florianv_swap.swap')]) plus the container-fetch fallback.
• Added a Per-query options table.
• Replaced the legacy Supported Services code dump with a pointer to the Swap README Providers table.
• Added a short FAQ at the end.

composer.json

• Updated description and Packagist keywords for discoverability.
• Fixed the homepage field, which still pointed at the old FlorianvSwapBundle URL.

LICENSE

• Added a top-level LICENSE file (MIT) so GitHub detects the license on the repository sidebar (the existing Resources/meta/LICENSE was not picked up by the GitHub license detector). Copyright range refreshed to 2014-2026.

Why

• A new Symfony developer running composer require florianv/swap-bundle, registering the bundle, and dropping a florianv_swap.yaml immediately gets a working Swap\Swap injection, with no commercial-API friction.
• High-intent terms (Symfony currency conversion, Symfony exchange rates, drop-in bundle, container service, config/packages) are now in headings, the opening paragraph, and the Packagist metadata.
• The README is a value-proposition page, not a duplicate of the Swap README. The Providers table lives in one place.
• GitHub now recognizes the MIT license on the repository, restoring a trust signal that was missing.

What did NOT change

• No runtime code changes. Public API of Florianv\SwapBundle\FlorianvSwapBundle, FlorianvSwapExtension, and the florianv_swap.swap / florianv_swap.builder container service IDs is unchanged.
• No changes to DependencyInjection/, Resources/config/, Tests/, CI, Psalm config, or php-cs-fixer config.
• The Resources/meta/LICENSE file is kept for backward compatibility; the new top-level LICENSE is what GitHub now reads.
• The CHANGELOG was not touched (this is doc work, not a release).