Added example with files-array notation.

Hi!

I'd been looking for some time for a way of compiling all less files from a given directory, until I found the solution [here on StackOverflow](http://stackoverflow.com/questions/15597839/grunt-replace-all-less-files-into-css-files/15600742#15600742).

I reckon it's common functionality, so I think it'd be good to add it to the README.

Cheers,

Val Waeselynck