Added docs to show that CommonClassesStrategy is not a part of our BC promise (#63)

Added docs to show that CommonClassesStrategy is not a part of our BC promise.

Author: Nyholm

src/Strategy/CommonClassesStrategy.php

@@ -18,6 +18,8 @@
 use Http\Adapter\Buzz\Client as Buzz;
 
 /**
+ * @internal
+ *
  * @author Tobias Nyholm <[email protected]>
  */
 final class CommonClassesStrategy implements DiscoveryStrategy