Update Setup section README.md to include a complete bzlmod example

[hazelnusse]

Is your feature request related to a problem? Please describe.
In trying to use rules_nixpkgs in a new bzlmod based project, I struggled to find a complete working example of rules_nixpkgs with bzlmod.

Describe the solution you'd like
Add a self contained example to the Setup section of README.md which illustrates the canonical use of rules_nixpkgs with bazel modules. My particular use case is C++, so it would be cool to see an example which for example, had a cc_binary with deps that referenced a C/C++ library provided via nixpkgs. I'm also not a nix expert, so something that doesn't immediately blow my stack would be ideal :)

Describe alternatives you've considered
Not using rules_nixpkgs until I can understand if it fully supports bazel modules.

Additional context
I have read most of https://github.com/tweag/rules_nixpkgs/blob/master/design/bzlmod/README.md and found it to be a bit overwhelming. There are also some examples of using bazel modules here:
https://github.com/tweag/rules_nixpkgs/tree/master?tab=readme-ov-file#with-with-bzlmod
https://github.com/tweag/rules_nixpkgs/tree/master?tab=readme-ov-file#bazel-7-with-bzlmod
but these are buried inside the documentation to nixpkgs_java_configure and include a bunch of other things that are irrelevant to non-Java use cases.

[oliverlee]

I am also not a nix expert, but here's an example of linking a C++ binary to LAPACK, which is provided by nix.
https://github.com/oliverlee/rules_nix_flake_example

LAPACK is a split package with the pre-built libraries in lapack.out and headers in lapack.dev -- these are declared as outputs in flake.nix. extensions.bzl defines a module extension that pulls in these packages.

flake_lapack-out uses the default Bazel build file, which looks like this:

package(default_visibility = ["//visibility:public"])

filegroup(
    name = "bin",
    srcs = glob(["bin/*"], allow_empty = True),
)

filegroup(
    name = "lib",
    srcs = glob(["lib/**/*.so*", "lib/**/*.dylib", "lib/**/*.a"], allow_empty = True),
)

filegroup(
    name = "include",
    srcs = glob(["include/**/*.h", "include/**/*.hh", "include/**/*.hpp", "include/**/*.hxx"], allow_empty = True),
)

Instead of using the default build file for lapack-dev, we provide a custom build file that defines a cc_library target. srcs references the lib filegroup from @flake_lapack-out.

load("@rules_cc//cc:defs.bzl", "cc_library")

cc_library(
    name = "lapack",
    hdrs = glob(
        [
            "include/**/*.h",
        ],
        allow_empty = False,
    ),
    srcs = ["@flake_lapack-out//:lib"],
    includes = ["include"],
    visibility = ["//visibility:public"],
)

You can inspect the contents of the external repos with

cd $(bazel info output_base)/external/
ls -al +flake_package_deps+flake_lapack-out/
ls -al +flake_package_deps+lapack/

The directory names may differ depending on your version of Bazel and other options.

You can run the example with

bazel run //:example