README: Describe how to install nightly with a component easily

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

Author: kinnison

README.md

@@ -41,7 +41,7 @@ And it runs on all platforms Rust supports, including Windows.
 
 ## Installation
 
-Follow the instructions at https://rustup.rs. If
+Follow the instructions at <https://www.rust-lang.org/tools/install>. If
 that doesn't work for you there are [other installation
 methods](#other-installation-methods).
 
@@ -151,6 +151,26 @@ using the toolchain.
 [Environment Variables]: #environment-variables
 [cargo]: https://github.com/rust-lang/cargo
 
+#### Installing nightly
+
+When `rustup-init` installs the initial toolchain it _forces_ the installation
+and so will install the `nightly` channel regardless of whether it might be
+missing components that you want.  For example, if you want to make a fresh
+installation of `rustup` and then install `nightly` along with `clippy` or
+`miri` you will need to do this in two phases.
+
+Firstly install `rustup` by means of:
+
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -- --default-toolchain none -y
+
+Next you can install `nightly` allowing `rustup` to downgrade until it finds
+the components you need:
+
+    rustup toolchain install nightly --allow-downgrade --profile minimal --component clippy
+
+This can be used to great effect in CI, to get you a toolchain rapidly which meets
+your criteria
+
 ## How rustup works
 
 `rustup` is a *toolchain multiplexer*. It installs and manages many