docs: replace stale PHP RPC README

[manuelsampedro1]

The README currently describes a duplicated Go bgld client, including go get, Ginkgo, GoDoc, and Toorop/go-bitcoind references. That does not match this PHP Bitgesell Core RPC client and gives users the wrong setup path.

Changes:

• replace the stale duplicated Go README with PHP-specific project documentation
• add Composer source-checkout setup instructions
• add a minimal PHP client usage example for Bitgesell Core RPC
• document async usage, available connection options, and the PHPUnit command

Validation:

• reviewed the README for remaining stale Go/bitcoind references with rg
• git diff --check

Note: this is a docs-only change, so I did not run the PHP test suite locally.

Bounty context: BitgesellOfficial/bitgesell#39

[MyTH-zyxeon]

Maintainer-facing review assist for the Bitgesell RPC docs lane:

The README replacement is the right direction; it removes the duplicated Go client docs and gives this PHP package its own setup path. I would treat these two checks as the merge gate so users do not get a broken copy/paste path:

1. Align the README example with Composer autoload/package metadata. At head 656ee45, the README imports naftalimurgor\Bitgesell\Client, while composer.json still maps only Denpa\Bitcoin\ and still names/describes denpa/php-bitcoinrpc. A fresh composer install path will not autoload the README's class unless the PSR-4 mapping/package metadata is updated, or the README uses the actually autoloaded namespace.
2. Add one no-network smoke check to validation, for example composer dump-autoload plus a php -r 'require "vendor/autoload.php"; new \\naftalimurgor\\Bitgesell\\Client([...]);' style instantiation. That proves the README sample boots without requiring a live BGLd node.

Optional cleanup: the source/test phpdoc still carries some inherited Bitcoin Core / BitcoindResponse naming. That does not have to block this README-only PR, but calling it out as follow-up scope would avoid confusing package consumers.

No secret, wallet, live RPC, or node interaction is needed for this review path; this can stay documentation/autoload verification only.