Ch. 01: Show how to work offline

Fixes #3279

Author: chriskrycho

src/ch01-01-installation.md

@@ -142,6 +142,26 @@ integrated development environments (IDEs) have built-in support for Rust. You
 can always find a fairly current list of many editors and IDEs on [the tools
 page][tools] on the Rust website.
 
+### Working Offline with This Book
+
+In several examples, we will use Rust packages beyond the standard library. To
+work through those examples, you will either need to have an internet connection
+or to have downloaded those dependencies ahead of time. To download the
+dependencies ahead of time, you can run the following commands. (We’ll explain
+what `cargo` is and what each of these commands does in detail later.)
+
+```console
+$ cargo new get-dependencies
+$ cd get-dependencies
+$ cargo add [email protected] [email protected]
+```
+
+This will cache the downloads for these packages so you will not need to
+download them later. Once you have run this command, you do not need to keep the
+`get-dependencies` folder. If you have run this command, you can use the
+`--offline` flag with all `cargo` commands in the rest of the book to use these
+cached versions instead of attempting to use the network. 
+
 [otherinstall]: https://forge.rust-lang.org/infra/other-installation-methods.html
 [install]: https://www.rust-lang.org/tools/install
 [msvc]: https://rust-lang.github.io/rustup/installation/windows-msvc.html