Add NotePlan

[camflint]

Put "Add ____" as Title
[x] Done

Provide a link to the proposed addition.
https://noteplan.co/

Explain why it should be added
NotePlan is in the vein of Agenda and NoteShip, which are already listed here, only a bit more toward the "pure markdown" and "personal knowledge management" end of the spectrum. It's mac-native and quite fully-featured.

Additional context

Issue Checklist

Put an x in the boxes once you've completed each step. You can also fill these out after creating the issue. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before responding to the issue.

• I have checked for other similar issues
• I have explained why this change is important
• I have added necessary documentation (if appropriate)