add a section explaining how to run unit tests (#894)

docs/developpers.rst

@@ -140,3 +140,21 @@ Points of attention:
 * don't forget to declare the namespace: ``namespace captcha;``
 * don't forget to write the corresponding unit tests (see tests/InternalCaptchaTest.php)
 
+
+Run unit tests
+--------------
+
+Run the unit tests with this command:
+
+```
+XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-text --configuration tests/phpunit.xml
+```
+
+Take care to use the phpunit shipped with composer.
+
+If you don't have the composer dependencies yet:
+
+```
+composer update
+```
+