Update README

README.md

@@ -18,4 +18,28 @@ nix-shell «path-to-this-repo» -A genode.base
 
 Bootstrapping the environment on a laptop can take hours.
 
-TODO: easy binary cache configuration
+## Continuous builds
+
+### C3D2 hydra
+
+Hardware for continuous builds provided by [Chaos Computer Club Dresden](https://c3d2.de/).
+
+The first hydra is located at https://hydra.hq.c3d2.de/project/genodepkgs.
+
+The following NixOS module will enable use of this Hydra as a cache:
+
+```nix
+{ ... }: {
+  nix = {
+    trustedUsers = [ «your-user»  ];
+
+    trustedBinaryCaches = [
+      "https://nix-serve.hq.c3d2.de"
+    ];
+
+    binaryCachePublicKeys = [
+      "nix-serve.hq.c3d2.de:FEi9GyFkou1Ua8INaEKmuGaww9E5y3XwrNGNRfKYeLo="
+    ];
+  };
+}
+```