Update README.md

Add an info how to build remotely.

README.md

@@ -62,6 +62,13 @@ This will generate and run a deployment script, which does the following:
   - You can read more about the possible modes and parameters in the [nixos-rebuild manual page](https://www.mankier.com/8/nixos-rebuild)
 - Copy secrets from the local password store to `/var/src/secrets/` on the target
 
+**But building takes sooo long >:(**
+You can offload the build to a remote host, for example krul:
+
+```
+$ nix build --builders 'ssh://USERNAME@krul.finf.uni-hannover.de' -f . deploy.HOSTNAME
+```
+
 > If you need to set extra ssh parameters (eg. path to ssh-keyfile) you can specifiy them by exporting the `NIX_SSHOPTS` environment-variable with the needed ssh parameters.
 > This is useful (and necessary) when you don’t have your ssh-key on a YubiKey.
 > Example: `export NIX_SSHOPTS="-i ~/.ssh/id_rsa4096"`