Added @api tag for documentation

src/Spout/Reader/ReaderFactory.php

@@ -18,6 +18,7 @@ class ReaderFactory
     /**
      * This creates an instance of the appropriate reader, given the type of the file to be read
      *
+     * @api
      * @param  string $readerType Type of the reader to instantiate
      * @return ReaderInterface
      * @throws \Box\Spout\Common\Exception\UnsupportedTypeException