To add a new login method you have to go to the Settings tab. You can find if you click on the ≡ icon under your profile.
Once you are there, scroll down and you will see the ''Account connect tab''. Then follow the instructions to add a Facebook or a Google account to your profile.
That way you will be able to use this login method and log in to the game from different devices with your Facebook or Google account credentials using the appropriate connection button.