doc: test component

Author: soyuka

CONTRIBUTING.md

@@ -181,6 +181,11 @@ If you need a `Given` step, add it to the doctrine context in `tests/Core/Behat/
 
 The last step is to add you feature inside `features/`. You can add your test in one of our existing features, or create your own.
 
+## Components tests
+
+API Platform is split into several components. There are tests for each of these, to run them `cd src/Doctrine/Common` then `composer update` and `./vendor/bin/phpunit`.
+We do not provide a way to run all these tests at once yet.
+
 # License and Copyright Attribution
 
 When you open a Pull Request to the API Platform project, you agree to license your code under the [MIT license](LICENSE)