Add reference to Rosetta Code resource

[Jach]

Was having a derp moment earlier trying to remember a function, and googling provided me a reference to a rosetta code example (just showcasing/reminding me of the existence of alexandria:rotate), out of curiosity checked the cookbook home page and didn't see Rosetta Code linked. It was a very valuable resource for me when first learning the language and still comes in handy. Some solutions could probably use some TLC but the ones I've seen tend to be reasonably modern, with people quickloading packages as appropriate and so on, rather than trying to hardcore do everything in pure ANSI CL.