Fix wiki link in README

The wiki link in the README currently links to a 404, as GitHub transforms the current path into a URL for the blob of the wiki, which doesn't exist. This may be due to GitHub's [new support of relative links](https://github.com/blog/1395-relative-links-in-markup-files).

Using a full URL resolves this. There may be a better solution that preserves the relativeness of the path; if anyone knows of one, definitely chime in!