docs: document feature variation packages

Author: kittywitch

README.md

@@ -16,3 +16,13 @@ cargo build
 # Regenerate GitHub actions from changed ci.nix
 CI_PLATFORM=impure nix run -f https://github.com/arcnmx/ci/archive/v0.7.tar.gz run.gh-actions-generate --arg config ./ci.nix
 ```
+
+## Package variants (e.g. features)
+
+package.nix supports a `features` parameter. If you define a package with features, you can use the variant in both the outputs and within CI to provide builds with certain features.
+```
+        exampleWithFeatures = pkgs.callPackage ./package.nix {
+          craneLib = packageCraneLib;
+          features = [ "lots-of-spaghetti" ];
+        };
+```