Add exclusion pattern for .private directory

Author: PDowney

phpmd.xml

@@ -9,6 +9,7 @@
 
     <!-- WordPress-specific file patterns to exclude -->
     <exclude-pattern>*/vendor/*</exclude-pattern>
+    <exclude-pattern>*/.private/*</exclude-pattern>
     <exclude-pattern>*/node_modules/*</exclude-pattern>
     <exclude-pattern>*/tests/*</exclude-pattern>
     <exclude-pattern>*/assets/*</exclude-pattern>