How would I go about making a workflow that watches a particular folder, or any of its subfolders, for added files, and then appends a message to a text file that says "[filename] was added to the [folder] folder on [date]?
Keyboard Maestro has no facility for watching a folder and its subfolders. Probably you can use Hazel for that.
Watching a folder is easy. Watching subfolders in a folder is not easy.
Apple has rewritten AppleScript Folder-Actions for El Capitan, so this might be easy when it rolls around – but I won’t know until I can install and test.