@Ron As far as I know, there are free versions for all platforms. You can pay to get early access to new versions, and there are subscriptions for their publishing feature (to publish notes to the web) and a note syncing service, but otherwise all features are accessible in the free version. You can do advanced stuff with it, but basically you just create a new note, writing in Markdown (as you do here) and if you want to link to another note you type double square brackets and start typing the name of the note you want to link to. Give it a try and see if you like it!