Update README to use --locked for installing mdbook

[undecidedapollo]

Issue

When installing mdbook using the README command using the version referenced 0.4.28 an error is thrown due to some of mdbooks dependencies depending on a newer toolchain (Rust 1.70.0):

$ cargo install mdbook --version 0.4.28
    Updating crates.io index
  Installing mdbook v0.4.28
error: failed to compile `mdbook v0.4.28`, intermediate artifacts can be found at `/var/folders/f7/t0k51r194wn4_2j6j9bgd2nm0000gn/T/cargo-installVwT6UO`

Caused by:
  package `clap_complete v4.4.9` cannot be built because it requires rustc 1.70.0 or newer, while the currently active rustc version is 1.67.1
  Try re-running cargo install with `--locked`

Solution

Since the book is using the toolchain for Rust 1.67 and some of mdbook's dependencies are unpinned, we should include the --locked command to use the dependencies that were valid at the time of 0.4.28's publish which are compatible with Rust 1.67.

$ cargo install mdbook --locked --version 0.4.28
...
   Installed package `mdbook v0.4.28` (executable `mdbook`)

Note

I'm new to Rust and this is my first PR for the book. I'm not sure if there is a better solution to this vs. using --locked. When using locked, you get some warnings about yanked packaged (which is unfortunate):

warning: package `crossbeam-channel v0.5.6` in Cargo.lock is yanked in registry `crates-io`, consider running without --locked

Looking at issues w/ cargo it seems it doesn't have logic yet to find the best compatible version of a package for your toolchain: rust-lang/cargo#10903

Another option is updating the toolchain for mdbook to the latest version of rust. If that is the preferred solution I'd be glad to give that a try.

[chriskrycho]

I think we probably want both --locked and an updated version, so this seems good. Thank you!