Add .dropboxignore directory to exclude folders without using selective sync

@Johnny W.

My Dropbox client (v3.8.8 on Mac Os X 10.10.5) does delete from the HD the folders unchecked with Selective Sync. It actually warns me about this with a message:

http://www.guidowalterpettinari.eu/external_access/dropbox_selective_sync_msg.png 

I thought this was the major limitations of using Selective Synch, together with being unable to specify RegEx or glob patterns.