Add a hint how to detect Vips is correctly installed

[alexander-schranz]

No description provided.

[kleisauke]

FWIW, this can probably be simplified to just:

$ php -r "require_once('vendor/autoload.php'); echo 'libvips version: ' . \\Jcupitt\\Vips\\Config::version() . PHP_EOL;"
libvips version: 8.16.0

Since almost all checks mentioned in this PR are covered here:

php-vips/src/FFI.php

Lines 239 to 249 in a8cc66c

	// detect the most common installation problems
	if (!extension_loaded('ffi')) {
	throw new Exception('FFI extension not loaded');
	}
	if (!ini_get('ffi.enable')) {
	throw new Exception("ffi.enable not set to 'true'");
	}
	if (version_compare(PHP_VERSION, '8.3', '>=') &&
	ini_get('zend.max_allowed_stack_size') != '-1') {
	throw new Exception("zend.max_allowed_stack_size not set to '-1'");
	}

For example, when I remove the zend.max_allowed_stack_size = -1 setting in php.ini, I see:

$ php -r "require_once('vendor/autoload.php'); echo 'libvips version: ' . \\Jcupitt\\Vips\\Config::version() . PHP_EOL;"
PHP Fatal error:  Uncaught Jcupitt\Vips\Exception: zend.max_allowed_stack_size not set to '-1' in /home/kleisauke/php-vips/src/FFI.php:248
Stack trace:
#0 /home/kleisauke/php-vips/src/FFI.php(150): Jcupitt\Vips\FFI::init()
#1 /home/kleisauke/php-vips/src/Config.php(149): Jcupitt\Vips\FFI::version()
#2 Command line code(1): Jcupitt\Vips\Config::version()
#3 {main}
  thrown in /home/kleisauke/php-vips/src/FFI.php on line 248