Merge pull request #4138 from rust-lang/code-style-updates-Here

Document use of rustfmt and dprint for formatting

Author: chriskrycho

CONTRIBUTING.md

@@ -11,6 +11,35 @@ of the print version. The snapshot files reflect what has been sent or not, so
 they only get updated when edits are sent to No Starch. **Do not submit pull
 requests changing files in the `nostarch` directory, they will be closed.**
 
+We use [`rustfmt`][rustfmt] to apply standard formatting to Rust code in the
+repo and [`dprint`][dprint] to apply standing formatting to the Markdown source
+and the non-Rust code in the project.
+
+[rustfmt]: https://github.com/rust-lang/rustfmt
+[dprint]: https://dprint.dev
+
+You will normally have `rustfmt` installed if you have a Rust toolchain
+installed; if for some reason you do not have a copy of `rustfmt`, you can add
+it by running the following command:
+
+```sh
+rustup component add rustfmt
+```
+
+To install `dprint`, you can run the following command:
+
+```sh
+cargo install dprint
+```
+
+Or follow the [instructions][install-dprint] on the `dprint` website.
+
+[install-dprint]: https://dprint.dev/install/
+
+To format Rust code, you can run `rustfmt <path to file>`, and to format other
+files, you can pass `dprint <path to file>`. Many text editors also have native
+support or extensions for both `rustfmt` and `dprint`.
+
 ## Checking for Fixes
 
 The book rides the Rust release trains. Therefore, if you see a problem on