added note for sourceforge.jp urls in CONTRIBUTING

Does anyone have an idea what that `$NUMBER` (refraining from calling it `$INTEGER` for less technical users) part is? It seems to be in all urls of this type, and fairly specific to each download (meaning there’s no logical way to describe it, as it’s just something that increments on sourceforge’s servers).

@troter You were the one to add this exception. Any idea if it’s something we can be more descriptive about?