How to cite a specific living document in a GitHub repository?
Please go ahead a create a DOI for the specific release yourself and use it to cite the work.

Also, you could fork your own copy of the wiki and keep it updated from time to time from the main branch. 

Versioning DOI: You can also get a DOI automatically issued for every release of the 'Living Document' by Zenoda. Details are here.

Here is a guide: https://help.github.com/articles/referencing-and-citing-content/