Document how to run the pre-commit-test target natively

Author: greg0ire

docs/contributions/index.rst

@@ -10,5 +10,13 @@ Clone the mono repository
 Run the tests
 =============
 
+The project comes with a Makefile that will run the tests and other
+checks for you.
+It relies on ``docker``, but you can run any make target natively by
+using the ``PHP_BIN`` make variable.
+For instance, to run the pre-commit checks, you can run::
+
+    make PHP_BIN=php pre-commit-test
+
 Submit a Pull Request
 =====================