Add .dropboxignore directory to exclude folders without using selective sync

@Ben C.21

I did not do adequate research. The selective sync dialog says "If your hard drive is full, you don't necessarily need to delete files. If you have enough Dropbox space, you can instead use selective sync" and, because the opposite is not explicitly mentioned anywhere obvious, I stopped looking.

If anyone else is still confused, here's the roundabout way I used the dropbox GUI to stop syncing a folder:

1. Move everything inside the target folder to some other folder outside of dropbox

2. Open the selective sync dialog, select the target folder, hit OK. This folder will be deleted.

3. Create a new folder of the same name in the same location, move everything back into this folder

Buuutttt since you poked fun at developers for not being able to figure this out, I did what I should have done a long time ago: `man dropbox` and lo and behold the dropbox command line interface comes complete with beautifully simple exclude functionality!

From the command line the solution is super simple: `dropbox exclude add /path/to/folder` (as mentioned here: https://help.dropbox.com/syncing-uploads/selective-sync-overview .. again, I clearly didn't do enough research before jumping to conclusions)

Thank you Ben C.21, my love for dropbox has been restored!