message: Fix links in the guide
  Add a link to a missing page so it's rendered
diff: * [Cargo.toml vs Cargo.lock](guide/cargo-toml-vs-cargo-lock.md)
  * [Tests](guide/tests.md)
  * [Continuous Integration](guide/continuous-integration.md)
  + * [Build Cache](guide/build-cache.md)
  * [Cargo Reference](reference/index.md)
  * [Specifying Dependencies](reference/specifying-dependencies.md)
  