Add .dropboxignore directory to exclude folders without using selective sync

So if you're working on local versions of websites that uses some CMSs that create cache files, these often don't have the right permissions to be uploaded or indexed by dropbox and it causes it to be mega slow. 

The best way to stop this is to have a .ignore file like git, also if you have local git repositories, you don't need to index the .git files in dropbox.

So need this. now. please.