content: Add installation guide of rustfmt

Aside from stable `rustfmt` guide, add nightly `rustfmt` installation
and use guide to format Rust code with unstable options.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

Author: RuoqingHe

src/repo-management/firecracker/rustfmt.md

@@ -12,3 +12,33 @@ their CI.
 This script lints every staged file, as of `.rs`s, it apply `cargo fmt` on them.
 The command directly formats that file instead of produce differences
 before/after format.
+
+## Install rustfmt
+
+`rustfmt` (invoked by `cargo fmt` command) could be installed with:
+
+```sh
+rustup component add rustfmt
+```
+
+And we can then use it with command:
+
+```sh
+cargo fmt
+```
+
+### Nightly Toolchain
+
+Sometimes we need unstable options of `rustfmt`, e.g. `group_imports` and the
+like. These options are not available in stable channel, thus we need to install
+nightly channel:
+
+```sh
+rustup component add rustfmt --toolchain nightly
+```
+
+Then we can use nightly `rustfmt` with:
+
+```sh
+cargo +nightly fmt
+```