Marcin, this is great! I love Sidenote already, just started using it today, and already purchased a license.
I have an awesome suggestion that would make it even more powerful, and useful. Esp while you build out the iCloud sync part. Right now, in the preferences you can do "Data" and then "Open Data Folder" to see the info. It would be INCREDIBLE if you could change that location. So I could put my Sidenote data in a Dropbox folder, so then it would be accessible on other computers, and backed up as well. The same would apply for people to put that in like Google Drive folders, or other syncing software like that. Please consider this feature, it would be a HUGE addition to the app, allowing users to customize the location of that "Data folder."
I think that for now you could try to link data folder with a folder in iCloud / DropBox. We'll think about it. We need to figure out which method of iCloud synchronisation to choose.