Add a note about using data providers to the CONTRIBUTING.md guide

PHPCS convention is to define data providers below the test method and to name them using the "data" prefix. This commit is an attempt to document this convention in the CONTRIBUTING.md guide.

Author: rodrigoprimo

.github/CONTRIBUTING.md

@@ -384,6 +384,7 @@ To run the tests specific to the use of `PHP_CODESNIFFER_CBF === true`:
     In such cases, the `PHP_CodeSniffer\Tests\Core\Config\AbstractRealConfigTestCase` should be used as the base test class.
 * Tests for the `Runner` class often can't create their own `Config` object in the tests, so run into the same issue.
     Those tests should use the `PHP_CodeSniffer\Tests\Core\Runner\AbstractRunnerTestCase` base class, which will ensure the Config is clean.
+* When using data providers, define them immediately below the corresponding test method. The data provider's name should match the test method name, replacing the "test" prefix with "data". For example, the data provider for a method named `testSomething()` should be `dataSomething()`.
 
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