introducing CONTRIBUTING.md #694

Job	Run time
PHPUnit (PHP 7.2)	32s
PHPUnit coverage (PHP 7.4)	1m 3s
Tests alternative autoloader (PHP 7.1)	7s
PHPUnit (PHP 7.3)	33s
PHPUnit Composer v2 (PHP 7.4)	28s
Tests alternative autoloader (PHP 7.2)	11s
PHPUnit (PHP 7.4)	35s
Tests alternative autoloader (PHP 7.3)	10s
PHPUnit (PHP 8.0)	31s
PHPUnit (PHP 7.1)	38s
PHPUnit (PHP 8.1)	38s
PHPUnit lowest deps (PHP 7.3)	29s
Tests alternative autoloader (PHP 7.4)	8s
Tests alternative autoloader (PHP 8.0)	24s
Tests alternative autoloader (PHP 8.1)	7s
Windows-Tests with PHP 7.1	1m 28s
Tests alternative autoloader (PHP 8.2)	8s
Windows-Tests with PHP 7.2	1m 3s
Tests alternative autoloader (PHP 8.3)	18s
PHPUnit (PHP 8.2)	35s
Windows-Tests with PHP 7.3	55s
Tests alternative autoloader (PHP 8.4)	8s
PHPUnit (PHP 8.3)	37s
Windows-Tests with PHP 7.4	1m 7s
PHPUnit (PHP 8.4)	38s
Windows-Tests with PHP 8.0	56s
Windows-Tests with PHP 8.1	59s
Windows-Tests with PHP 8.2	57s
Windows-Tests with PHP 8.3	53s
Windows-Tests with PHP 8.4	50s
	18m 6s

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