Update readme to add toolchain installation instruction

The previous commit automates toolchain installation for the CI, this
commit reflect the changes in the readme by providing instructions for
installing the appropriate toolchain.

readme.md

@@ -8,6 +8,7 @@ The Mirage project uses [`just`](https://github.com/casey/just) to easily build,
 You can easily install `just` with your favorite package manager or `cargo` by following [the instructions](https://just.systems/man/en/chapter_4.html).
 
 Mirage is primary developed and tested on QEMU, therefore you will need to install `qemu-system-riscv64` on your system.
+Then you will need to install the rust toolchain, if rust is installed through rustup on the machine this can be done by running `just install-toolchain`
 
 Then running Mirage is as simple as invoking `just run`.