message: Add link to docs.rs to the README.
diff: |
  <a href="https://bheisler.github.io/criterion.rs/book/index.html">User Guide</a>
  |
  - <a href="https://bheisler.github.io/criterion.rs/criterion/">API Documentation</a>
  + <a href="https://bheisler.github.io/criterion.rs/criterion/">Master API Docs</a>
  + |
  + <a href="https://docs.rs/crate/criterion/">Released API Docs</a>
  |
  <a href="https://github.com/bheisler/criterion.rs/blob/master/CHANGELOG.md">Changelog</a>
  </div>
  