Add .dropboxignore directory to exclude folders without using selective sync

well, I can tell you it is more than 8 years, since it has been first requsted to get an option to exclude files/folders (this was in the old forum before this one, might have been even earlier, it was for sure before the votebox interlude)
anyway, at that time Arash made a comment they have it internally, in it will soon be public, well there we are many many many years later, and still nothing
(luckly for dropbox the forum is gone, so we cannot show them for how long this is outstanding)