Document cross-compilation on macOS

The instructions in the book are now out of date for projects generated
with this template. Since the instructions are much shorter these days,
put them in the README.

README.md

@@ -2,7 +2,12 @@
 
 ## Prerequisites
 
-1. Install bpf-linker: `cargo install bpf-linker`
+1. stable rust toolchains: `rustup toolchain install stable`
+1. nightly rust toolchains: `rustup toolchain install nightly --component rust-src`
+1. (if cross-compiling) rustup target: `rustup target add ${ARCH}-unknown-linux-musl`
+1. (if cross-compiling) LLVM: (e.g.) `brew install llvm` (on macOS)
+1. (if cross-compiling) C toolchain: (e.g.) [`brew install filosottile/musl-cross/musl-cross`](https://github.com/FiloSottile/homebrew-musl-cross) (on macOS)
+1. bpf-linker: `cargo install bpf-linker` (`--no-default-features` on macOS)
 
 ## Build & Run
 
@@ -11,3 +16,16 @@ Use `cargo build`, `cargo check`, etc. as normal. Run your program with `xtask r
 Cargo build scripts are used to automatically build the eBPF correctly and include it in the
 program. When not using `xtask run`, eBPF code generation is skipped for a faster developer
 experience; this compromise necessitates the use of `xtask` to actually build the eBPF.
+
+## Cross-compiling on macOS
+
+Cross compilation should work on both Intel and Apple Silicon Macs.
+
+```bash
+AYA_BUILD_EBPF=true \
+CC=${ARCH}-linux-musl-gcc \
+RUSTFLAGS="-C linker=${ARCH}-linux-musl-gcc" \
+  cargo build --package {{project-name}} --release --target=${ARCH}-unknown-linux-musl
+```
+The cross-compiled program `target/${ARCH}-unknown-linux-musl/release/{{project-name}}` can be
+copied to a Linux server or VM and run there.