release.nix: Preface with a usage note
+# What's release.nix?
+# ===================
+#
+# This is mainly intended to be run by the build farm at the foundation's Hydra
+# instance. Though you can use it to run your builds, it is not as ergonomic as
+# using `nix-build` on `./default.nix`.
+#
# Note:
# Verify that .ci/instantiate-all.nix lists the expected paths when adding to this file.
let
