It’s a feature yet to be implemented - but should be available in June. 
 

Edited to add:

Sorry -that’s for queue adding, not playlist. 

If you have the app on another device that you have not updated, for me its my Mac, You can use that to edit your playlist, and queues, or even to update your music library if yo have music stored locally. Then you need to restart the mobile app to see the changes.