README.md: Broken link in List of available processing methods

In this section: https://github.com/carrierwaveuploader/carrierwave#list-of-available-processing-methods, the "[vary slightly between them](https://github.com/carrierwaveuploader/carrierwave/blob/master/supported-processing-methods)." link in the note is not found.