Added @api tag for documentation

src/Spout/Reader/AbstractReader.php

@@ -55,6 +55,7 @@ abstract class AbstractReader implements ReaderInterface
      * Prepares the reader to read the given file. It also makes sure
      * that the file exists and is readable.
      *
+     * @api
      * @param  string $filePath Path of the file to be read
      * @return void
      * @throws \Box\Spout\Common\Exception\IOException If the file at the given path does not exist, is not readable or is corrupted
@@ -92,6 +93,7 @@ abstract class AbstractReader implements ReaderInterface
     /**
      * Returns an iterator to iterate over sheets.
      *
+     * @api
      * @return \Iterator To iterate over sheets
      * @throws \Box\Spout\Reader\Exception\ReaderNotOpenedException If called before opening the reader
      */
@@ -107,6 +109,7 @@ abstract class AbstractReader implements ReaderInterface
     /**
      * Closes the reader, preventing any additional reading
      *
+     * @api
      * @return void
      */
     public function close()