Provide XSD for phpcs file (doctrine#11752)

It unlocks autocompletion and validation in some IDEs.

Author: greg0ire

phpcs.xml.dist

@@ -1,5 +1,7 @@
 <?xml version="1.0"?>
-<ruleset>
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         name="PHP_CodeSniffer"
+         xsi:noNamespaceSchemaLocation="vendor/squizlabs/php_codesniffer/phpcs.xsd">
     <arg name="basepath" value="."/>
     <arg name="extensions" value="php"/>
     <arg name="parallel" value="80"/>