Explain machines/*/secrets in readme

README.md

@@ -12,6 +12,9 @@
      for initial setup.
    + `services`: Non-trivial machine-specific configuration related to a
      specific service the machine provides.
+   + `secrets`: Nix expressions that include information that is not meant to
+     be visible to everyone (e.g. accounts, password hashes, private
+     information etc.)
  * `modules`: Custom modules. Many are activated by default, since I want them
    on all systems.
  * `pkgs`: My nixpkgs overlay