eval.nix: Adds meta.machinesFile option that is passed to Nix as builders argument

README.md

@@ -54,6 +54,15 @@ Here is a sample `hive.nix` with two nodes, with some common configurations appl
     nodeNixpkgs = {
       node-b = ./another-nixos-checkout;
     };
+
+    # If your Colmena host has nix configured to allow for remote builds
+    # (for nix-daemon, your user being included in trusted-users)
+    # you can set a machines file that will be passed to the underlying
+    # nix-store command during derivation realization as a builders option.
+    # For example, if you support multiple orginizations each with their own
+    # build machine(s) you can ensure that builds only take place on your
+    # local machine and/or the machines specified in this file.
+    # machinesFile = ./machines.client-a;
   };
 
   defaults = { pkgs, ... }: {