Specifying how to set toolchain to nightly for project can use rust-toolchain file

[ssnover]

The book currently indicates that the user should set their toolchain for the project using rustup override set nightly which does achieve the purpose locally. However if the project is moved or pushed to a git repository and cloned later, this information will be lost and not everyone will be savvy enough to know the toolchain needs to be nightly.

A better solution is to add a rust-toolchain file to the root of the project with echo "nightly" > rust-toolchain which will persist in the project.