Add documentation on how to export variable at runtime with set() and get()

**Your Godot version:**
3.2.3

**Issue description:**
I am trying to make a JSON resource importer but I'm not sure how to export dynamically created variables
Please add documentation on how to export variable at runtime with set() and get()

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/plugins/editor/import_plugins.html