There is already a paste button, although not directly shown. I can access it in 3 different ways (but I have made some changes to the default settings, so it might not be exactly the same for you):
on the toolbar on top of the keyboard (you need to extend it by tapping the > button on the top left). You can customise the buttons that show up here in "Settings/Preferences/Additional keys/Select toolbar keys"
by long pressing the comma key (my favourite way), a pop up shows with several keys, among them the paste key.