doc/contributing: rustfmt +nightly, prerequisites section (#2449)

* add(doc/contributing): rustfmt +nightly, prerequisites section

* update(doc/contributing): doctoc update

CONTRIBUTING.md

@@ -11,6 +11,8 @@ and introduce yourself.
 - [Code of Conduct](#code-of-conduct)
 - [Filing an Issue](#filing-an-issue)
 - [Looking to Start Contributing to `bindgen`?](#looking-to-start-contributing-to-bindgen)
+- [Prerequisites](#prerequisites)
+  - [`rustfmt` / `cargo fmt`](#rustfmt--cargo-fmt)
 - [Building](#building)
 - [Testing](#testing)
   - [Overview](#overview)
@@ -63,6 +65,16 @@ issue, provide us with:
 * [Issues labeled "help wanted"](https://github.com/rust-lang/rust-bindgen/labels/help%20wanted)
 * Still can't find something to work on? [Drop a comment here](https://github.com/rust-lang/rust-bindgen/issues/747)
 
+## Prerequisites
+
+### `rustfmt` / `cargo fmt`
+
+We use `nightly` channel for `rustfmt` so please set the appropriate setting your editor/IDE for that.
+
+For rust-analyzer, you can set `rustfmt.extraArgs = ['+nightly']`.
+
+To check via command line, you can run `cargo +nightly fmt --check`.
+
 ## Building
 
 To build the `bindgen` library and the `bindgen` executable: