Install the app and run it. This will register the availability of the app’s QuickLook extensions, Previewer and Thumbnailer. You can now quit the app: just click OK:
You can disable or re-enable Previewer and Thumbnailer in System Preferences > Extensions:
For some background information on how Catalina delivers QuickLook previews, please see this blog post.
If it appears that either Previewer or Thumbnailer are not working after you have run the host app, please log out of your Mac and then log back in. A future release will offer to do this for you.
If you have found PreviewMarkdown to be useful, please consider writing a positive review on the Mac App Store, or simply give it a rating.
In addition to its own code, PreviewMarkdown makes use of code from the following sources:
string index out of rangein SwiftyMarkdown).
You can view PreviewMarkdown’s source code at GitHub.