update readme

README.md

@@ -30,6 +30,25 @@ Included checks:
 }
 ```
 
+## Dependencies
+
+(Native) build dependencies can be specified with the following
+
+```nix
+{
+  # ...
+  outputs = { mill-scale, ... }: mill-scale ./. {
+    nativeBuildInputs = pkgs: [pkgs.libfoo];
+    buildInputs = pkgs: [pkgs.libbar];
+  };
+}
+```
+
+Additionally, dependencies for the following crates should be automatically detected
+
+- openssl
+- udev
+
 ## Usage with GitHub Actions
 
 ### Single runner
@@ -71,7 +90,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v27
       - id: set-matrix
-        run: echo "matrix={\"check\":$(nix eval --json '.#checks.x86_64-linux' --apply 'builtins.attrNames')}" | tee $GITHUB_OUTPUT
+        run: echo "matrix={\"check\":$(nix eval --json '.#checks.x86_64-linux' --apply 'builtins.attrNames')}" | tee -a $GITHUB_OUTPUT
 
   checks:
     runs-on: ubuntu-latest
@@ -87,6 +106,64 @@ jobs:
       - run: nix build .#checks.x86_64-linux.${{ matrix.check }}
 ```
 
+## Cross-compiling
+
+Cross-compiling can be enabled with the following
+
+```nix
+{
+  # ...
+  outputs = { mill-scale, ... }: mill-scale ./. {
+    crossTargets = [
+      "x86_64-unknown-linux-musl"
+      "x86_64-pc-windows-gnu"
+    ];
+  };
+}
+```
+
+This adds cross-compiled package with the same name as the target to the `packages`.
+
+### Cross-compiling with GitHub Action
+
+A `lib.crossMatrix` flake output is provided to help with setting up cross-compiling in CI.
+
+This contains a list of targets and the binary suffix (e.g. `.exe`) for each target.
+
+```yaml
+on: [push, pull_request]
+
+name: CI
+
+jobs:
+  matrix:
+    runs-on: ubuntu-latest
+    outputs:
+      cross-matrix: ${{ steps.set-matrix.outputs.cross-matrix }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+      - id: set-matrix
+        run: |
+          echo "cross-matrix={\"include\":$(nix eval --json '.#lib.crossMatrix')}" | tee -a $GITHUB_OUTPUT
+
+  build:
+    runs-on: ubuntu-latest
+    needs: [matrix]
+    strategy:
+      fail-fast: false
+      matrix: ${{fromJson(needs.matrix.outputs.cross-matrix)}}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        # insert cache setup here
+      - run: nix build .#${{ matrix.target }}
+      - uses: actions/upload-artifact@v4
+        with:
+          name: crate-name-${{ matrix.target }}
+          path: result/bin/crate-name${{ matrix.binary-suffix }}
+```
+
 ## Credits
 
 This flake is based on [flakelight-rust](https://github.com/accelbread/flakelight-rust), credit for most ideas got to accelbread.