Setup ReadTheDocs automatic build integration

[Matthew Peveler]

It would be beneficial to me (and probably others) if this repo was set-up to have automatic integration with ReadTheDocs such that pushes to master would then update https://open-recipe-format.readthedocs.io. As it stands for example, while #21 was merged, I am finding myself still relying on my fork at https://openrecipeformat.readthedocs.io to actually consult reference against which I would like to take down to have one definitive source.

Documentation on how to do this can be found at https://docs.readthedocs.io/en/stable/webhooks.html#github.