update github workflow description

.github/workflows/nix-github-actions.yml

@@ -1,3 +1,15 @@
+# this workflow automatically generates a matrix of builds and checks based on configuration in flake.nix
+# 
+# githubActions = nix-github-actions.lib.mkGithubMatrix {
+#  checks = {
+#    inherit (self.checks) x86_64-linux;
+#    x86_64-darwin = builtins.removeAttrs self.checks.x86_64-darwin ["formatting"];
+#  };
+#};
+#
+# runs all checks on linux and only the non-formatting checks on darwin
+#
+
 name: Nix Flake actions
 
 on: