update readme

README.MD

@@ -88,7 +88,7 @@ Adjust image opacity
 
 Node option:   
 * image: Image input, supporting RGB and RGBA. if is RGB, the alpha channel of the entire image will be automatically added.
-* mask: Mask input. Attention that if the image input has alpha channel and different masks are input simultaneously, the original alpha channel will be overwritten by the mask.
+* mask: Mask input. this is not a mandatory input item. attention that if the image input has alpha channel and different masks are input simultaneously, the original alpha channel will be overwritten by the mask.
 * invert_mask: Whether to reverse the mask.
 * opacity: Opacity of image.
 

README_CN.MD

@@ -83,7 +83,7 @@ ComfyUI
 
 节点选项说明：   
 * image: 图像输入，支持RGB和RGBA输入。如果输入的是RGB，将自动添加整个图像的alpha通道。
-* mask: 遮罩输入。注意如果image输入有alpha通道且同时输入了不同的mask，原本的alpha通道将被mask覆盖。
+* mask: 遮罩输入，这不是必须的输入项。注意如果image输入有alpha通道且同时输入了不同的mask，原本的alpha通道将被mask覆盖。
 * invert_mask: 是否反转遮罩。
 * opacity: 不透明度。