Use nix.settings.extra-* (#147)

README.md

@@ -125,12 +125,12 @@ You might also want to use the binary cache to avoid building locally.
 ```nix
 nix.settings = {
     builders-use-substitutes = true;
-    # substituters to use
-    substituters = [
+    # extra substituters to add
+    extra-substituters = [
         "https://anyrun.cachix.org"
     ];
 
-    trusted-public-keys = [
+    extra-trusted-public-keys = [
         "anyrun.cachix.org-1:pqBobmOjI7nKlsUMV25u9QHa9btJK65/C8vnO3p346s="
     ];
 };