These are useful links to other styleguides. This is worth clarifying as it's not clear whether/how they are maintained. Could consider removing them from here. I have not included the external link to the [wiki wiki](https://github.com/alphagov/wiki/wiki) for gem guidelines, as they should be here.
GitHub supports relative links now: <https://github.com/blog/1395-relative-links-in-markup-files>