Exclude directories and files from iCloud Drive

To exclude a directory or file from being synced to iCloud Drive, simply add .nosync to its name:

$ mv ~/Documents/secrets ~/Documents/secrets.nosync/

Some programs depend on directories and files having a specific name, with node_modules being a good example. To exclude these directories from syncing, without messing anything up, rename them to <directory name>.nosync and create a symlink without the .nosync part:

$ mv node_modules node_modules.nosync/ && ln -s node_modules.nosync node_modules