Added CONTRIBUTING.md file #2987

Job	Run time
PHPStan	37s
Tests ([email protected], AUTOLOAD=0)	1m 0s
Publish	0s
PHP-CS-Fixer	15s
Tests ([email protected], AUTOLOAD=0)	55s
Tests ([email protected], AUTOLOAD=0)	22s
Tests ([email protected], AUTOLOAD=0)	27s
Tests ([email protected], AUTOLOAD=0)	21s
Tests ([email protected], AUTOLOAD=0)	21s
Tests ([email protected], AUTOLOAD=0)	31s
Tests ([email protected], AUTOLOAD=0)	29s
Tests ([email protected], AUTOLOAD=0)	25s
Tests ([email protected], AUTOLOAD=1)	1m 5s
Tests ([email protected], AUTOLOAD=1)	53s
Tests ([email protected], AUTOLOAD=1)	25s
Tests ([email protected], AUTOLOAD=1)	22s
Tests ([email protected], AUTOLOAD=1)	24s
Tests ([email protected], AUTOLOAD=1)	23s
Tests ([email protected], AUTOLOAD=1)	37s
Tests ([email protected], AUTOLOAD=1)	45s
Tests ([email protected], AUTOLOAD=1)	32s
	11m 9s

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