Adding a non-existent link to ensure linkcheck works

Author: Michael-F-Bryan

src/about-this-guide.md

@@ -11,4 +11,6 @@ be found at [the GitHub repository]. If you find any mistakes in the
 guide, please file an issue about it -- or, even better, open a PR
 with a correction!
 
+[A non-existent link](foo.md)
+
 [the GitHub repository]: https://github.com/rust-lang-nursery/rustc-guide/