Add link for component availability
I always forget the links, so having it up
visible on rustup.rs is probably a good idea.
<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>
