Add CONTRIBUTING.md

[pronskiy]

The repo has no contribution guide. Create CONTRIBUTING.md covering:

• How to build: phpize && ./configure --enable-php-debugger && make
• How to run tests: TEST_PHP_ARGS='-dzend_extension=modules/php_debugger.so' php run-xdebug-tests.php -q tests/debugger/
• PR process: all changes via PRs, no direct pushes to main
• Code style: tabs for indentation (inherited from Xdebug)
• Test requirements: include .phpt tests for new features, no new XFAIL regressions
• Link to issue tracker for first-time contributors

Acceptance criteria

• CONTRIBUTING.md exists at repo root
• Build and test instructions are correct and tested