Introducing the Credentials plugin

Hello all, I created a Tutor plugin for the Credentials application for open-release/maple.3, located here: https://github.com/lpm0073/tutor-contrib-credentials.

Please note that I’d found this existing project – https://github.com/pcliupc/tutor-credentials – but it seems to be abandoned and so I created a new project using https://github.com/overhangio/cookiecutter-tutor-plugin.

Pull requests are welcome :smiley: