update watermark doc

Author: noogen

files/etc/nginx/sites-enabled/server.conf

@@ -246,6 +246,10 @@ server {
         image_filter                rotate $rotate;
 
         image_filter                resize $width $height;
+
+# use watermark directive last
+# image_filter watermark /path/to/watermark.png [ bottom-right (default) | top-left | top-right | bottom-left | center ];
+
     }
 
     location /cmd/crop {
@@ -267,6 +271,10 @@ server {
 
         image_filter_crop_offset    $crop_offx $crop_offy;
         image_filter                crop $width $height;
+
+# use watermark directive last
+# image_filter watermark /path/to/watermark.png [ bottom-right (default) | top-left | top-right | bottom-left | center ];
+
     }
 
     location @handle_redirect {