Document contributor certification in CONTRIBUTING.md [skip ci]

Author: bukka

CONTRIBUTING.md

@@ -32,6 +32,11 @@ had several contributions accepted, commit privileges are often quickly granted.
 PHP welcomes pull requests to [add tests](#writing-tests), fix bugs and to
 implement RFCs. Please be sure to include tests as appropriate!
 
+By submitting a pull request, you certify that you have the right to submit
+the work under the PHP License and, if applicable, that you have permission
+from your employer or organization. Your contribution will be licensed under
+the PHP License.
+
 If you are fixing a bug, then please submit your PR against the lowest actively
 supported branch of PHP that the bug affects (only green branches on
 [the supported version page](https://www.php.net/supported-versions.php) are