Add crop method usage to examples

[aalster]

It is a common use-case to crop an image to fit thumbnail size without leaving the gaps on sides. Currently examples page says

NOTE: Using this approach, the thumbnail will not necessarily be 200 pixel by 200 pixel dimensions

but doesn't provide a solution to force size keeping original aspect ratio.

Please add this example to Forcing thumbnails to be exactly the specified size section:

BufferedImage originalImage = ImageIO.read(new File("original.png"));

BufferedImage thumbnail = Thumbnails.of(originalImage)
        .crop(Positions.CENTER)
        .size(200, 200)
        .asBufferedImage();

This is a great feature of library but it is not easy to spot. Initially I was calculating sizes myself which required me to read bytes as BufferedImage to get original image size and then read bytes second time to make library determine output format automatically.

[coobird]

Thank you for the suggestion! That's a great idea!

[sparkwan]

I also need this feature @coobird.
Another feature: crop an transparent/white backgroud image automatically using a specify margin at orignal aspect ratio? The basic idea is that to find the start and end of X/Y cordinate by comparing pixel