Document how to do cross-compilation

The readme mentions how to cross compile for wasm, but doesn't explain cross compilation for any other platforms.

I am using rules_rust in conjunction with rules_docker and would like to compile with macOS as the host, targeting linux. Currently, without modifications, the docker image is build for macOS and fails on launch:

```
standard_init_linux.go:211: exec user process caused "exec format error"
```

My current attempt adds an entry to the .bazelrc file which specifies the platform for the image target.

```
build:image --platforms=//server:docker_platform
```

The platform is defined as such:

```
platform(
name = "docker_platform",
constraint_values = [
"@platforms//os:linux",
]
)
```

The error on build is

```
ERROR: While resolving toolchains for target //server:image_binary: no matching toolchains found for types @io_bazel_rules_rust//rust:toolchain
ERROR: Analysis of target '//server:image' failed; build aborted: no matching toolchains found for types @io_bazel_rules_rust//rust:toolchain
INFO: Elapsed time: 0.152s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded, 146 targets configured)
```

### Edit

I have loaded a custom toolchain in the WORKSPACE which seems like a step in the right direction but I am still hitting the same error.

```
rust_repository_set(
name = "rust_darwin_x86_64",
exec_triple = "x86_64-apple-darwin",
extra_target_triples = ["x86_64-unknown-linux-gnu"],
version = "1.39.0",
)
```

And updated .bazelrc, having tried both `@io_bazel_rules_rust//rust/platform:linux` and `//server:docker_platform` for the platforms argument

```
build:image --extra_toolchains=@rust_darwin_x86_64_toolchains//:toolchain_for_x86_64-unknown-linux-gnu --platforms=...
```