doc: cross-compile for aarch64

Reverse-engineer d'un dockerfile pas à jour, mais ça fonctionne sur Debian 12 x86_64 à destination d'un RPi 4 Debian 11 aarch64.

docs/user/build-for-arm.md

 # How to build duniter-v2s for arm
 
+Cross-compile Duniter to arm (e.g. Raspberry Pi).
+
+## With Docker
+
 1. Create a docker image that contains the build environment
 
 ```bash
@@ -13,3 +17,20 @@ docker build -t duniter-v2s-arm-builder -f docker/cross-arm.Dockerfile .
 ```
 
 then, get the final binary at `target/armv7-unknown-linux-gnueabihf/release/duniter`.
+
+## Without Docker
+
+**Warning**: armv7 (default for Raspberry Pi) is **not** supported. Linux on RPi can be easily switched to aarch64, please search how to do so on the Internet.
+
+This produces a musl build: the resulting executable is static, hence more portable than a dynamic one. It will be compatible with systems older than the compilation host.
+
+```bash
+# Install the tools
+rustup target add aarch64-unknown-linux-musl --toolchain nightly-2023-08-23-x86_64-unknown-linux-musl
+sudo dpkg --add-architecture arm64
+sudo apt update
+sudo apt install musl-dev:arm64 musl-tools g++-aarch64-linux-gnu gcc-aarch64-linux-gnu
+
+# Cross-compile
+CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_LINKER=aarch64-linux-gnu-gcc cargo build --target=aarch64-unknown-linux-musl --release
+```