message: update contributing to show how to build the docs
  The contributing guide referenced a script that was deleted in
  so it wasn't possible to follow the contributing guide
  successfully prior to contributing doc improvements.
diff: @@ -109,13 +109,16 @@ and [merges][mergequeue] it into Cargo's `master` branch.
  To contribute to the documentation, all you need to do is change the markdown
  files in the `src/doc` directory. To view the rendered version of changes you
  -have made locally, run:
  +have made locally, make sure you have `mdbook` installed and run:
  ```sh
  -sh src/ci/dox.sh
  -open target/doc/index.html
  +cd src/doc
  +mdbook build
  +open book/index.html
  ```
  +To install `mdbook` run `cargo install mdbook`.
  +
  ## Issue Triage
  