Rust 1.25.0

# Mutability

There is a new edition of the book and this is an old link.

> Variables are immutable only by default; we can make them mutable by adding mut in front of the variable name.

    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);Run

Here are the relevant sections in the new and old books:

- In the second edition: Ch 3.01 — Variables and Mutability
- In the first edition: Ch 3.11 — Mutability