message: Add components availability history link
  Some details on this dashboard:
diff: @@ -29,6 +29,7 @@ PRs against [rust-lang/rust-forge].
  * [The current PR testing queue](https://buildbot2.rust-lang.org/homu/queue/rust)
  * [The PR queue over time](https://rust-lang-nursery.github.io/rustc-pr-tracking/)
  * [Toolstate (rls, rustfmt, clippy)](https://rust-lang-nursery.github.io/rust-toolstate/)
  +* [Components availability history](https://rust-lang.github.io/rustup-components-history/index.html)
  * [Release history](releases.html). Links to previous release artifacts.
  * [Platform support](platform-support.html).
  * [Release process](release-process.html). How to make releases of Rust.
  