Add a requirement in tests installation instructions

PHPUnit requires Yaml package from pear.symfony.com channel.
diff --git a/tests/README.md b/tests/README.md
index a5f89a2..3e32edc 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -17,6 +17,7 @@
 PHP Unit >= 3.5.6
 
 	pear channel-discover pear.phpunit.de
+	pear channel-discover pear.symfony.com
 	pear install phpunit/PHPUnit
 
 vfsStream