Improve format and clarity in README.md

[fadrian06]

This pull request updates the README.md to clarify installation instructions and improve the explanation of PHP version support. The most important changes are:

Documentation improvements:

• Updated the example for manual installation to require flight/autoload.php instead of vendor/autoload.php or flight/Flight.php, simplifying setup instructions.

PHP version support clarification:

• Replaced the previous note about PHP 7.4 and 8 support with a clearer, formatted note block, specifying that PHP 7.4 is supported for compatibility with LTS distributions and that PHP 8 is also supported.

[Copilot]

Pull request overview

This pull request refines README.md to improve the installation example, tidy benchmark table formatting, and clarify PHP version support rationale for FlightPHP/Core.

Changes:

• Updated the “basic index.php” example to use flight/autoload.php.
• Normalized whitespace/alignment in the benchmark table.
• Replaced the prior PHP version support paragraph with an “IMPORTANT/NOTE” block format.

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