Added hint about duplicates

If one adds the same path to `sprockets` multiple times via `#append_path`, every resource under this directory will be added to the `middleman`'s sitemap as often as the source directory is included in the `#appended_paths`-list. I added a warning to the `middleman`-guide. 

As `Pathname`s are also supported, I added a short example for this as well.