Added tile size limit info

Author: tumic0

README.md

@@ -37,7 +37,8 @@ QImageReader reader(file, QByteArray::number(zoom));
 reader.setScaledSize(QSize(1024, 1024));
 reader.read(&img);
 ```
-you will get 1024x1024px tiles with a pixel ratio of 2 (= HiDPI tiles).
+you will get 1024x1024px tiles with a pixel ratio of 2 (= HiDPI tiles). The maximal
+tile size is 4096x4096.
 
 ### Overzoom
 Since version 3 of the plugin tile overzoom is supported. If you set *format*