add CS-Fixer configuration

.gitattributes

@@ -0,0 +1,4 @@
+* text=auto
+
+/.gitattributes export-ignore
+/.php-cs-fixer.php export-ignore

.php-cs-fixer.php

@@ -0,0 +1,13 @@
+<?php
+
+$finder = PhpCsFixer\Finder::create()
+    ->in(__DIR__)
+;
+
+$config = new PhpCsFixer\Config();
+return $config->setRules([
+        '@PHP73Migration' => true,
+        '@PSR12' => true
+    ])
+    ->setFinder($finder)
+;