gem omniauth-github:
https://github.com/omniauth/omniauth-github
Devise + Omniauth integration - basic docs:
https://github.com/zquestz/omniauth-google-oauth2#devise
Gem agains Authenticity error:
https://github.com/cookpad/omniauth-rails_csrf_protection
Ultimate text guide:
https://blog.corsego.com/devise-omniauth-github
Source code:
https://github.com/corsego/29-omniauth/commit/dc435261173dd3b8bac22971f4ab9daf86842dd9