apple-macbook-pro-8-1: add README.md

explaning to users how they can enable installtion of free packages
needed for b43-firmware to work

apple/macbook-pro/8-1/README.md

@@ -0,0 +1,24 @@
+# MacBook Pro 8,1
+
+## Enable unfree packages in your nix config for b43-firmware (wifi driver) to work
+
+### For b43-firmware only (Ideal)
+
+```nix
+{lib, ...}:
+
+{
+  nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+    "b43-firmware"
+  ];
+}
+
+```
+
+### For all packages
+
+```nix
+{
+  nixpkgs.config.allowUnfree = true;
+}
+```