Merge pull request #518 from sophiebits/patch-1

Mention `rustup upgrade` in install instructions

Author: chriskrycho

templates/tools/install.hbs

@@ -39,6 +39,10 @@
         beta and nightly release channels as well as support for additional
         cross-compilation targets.
       </p>
+      <p>
+        If you've installed <code>rustup</code> in the past, you can update
+        your installation by running <code>rustup upgrade</code>.
+      </p>
       <p>
         For more information see the
         <a href="https://github.com/rust-lang-nursery/rustup.rs/blob/master/README.md" target="_blank" rel="noopener"><code>rustup</code>