Add link for component availability

I always forget the links, so having it up
visible on rustup.rs is probably a good idea.

Author: Berkus Karchebnyy

www/index.html

@@ -134,6 +134,8 @@
   <br/>
   <a href="https://github.com/rust-lang/rustup.rs/#other-installation-methods">other installation options</a>
   &nbsp;&middot;&nbsp;
+  <a href="https://rust-lang-nursery.github.io/rust-toolstate/">component availability</a>
+  &nbsp;&middot;&nbsp;
   <a href="https://github.com/rust-lang/rustup.rs">about rustup</a>
 </p>