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