update readme

its incompleteness annoys me

README.md

@@ -10,6 +10,8 @@ welcome to hacc nixfiles (haccfiles). this is the code describing our nix-based
 - `modules/`: home-grown modules for hacc-specific services
 - `nix/`: sources files, managed with niv
 - `pkgs/`: packages we built and don't want to upstream
+- `hosts/`: configuration.nix per host
+- `services/`: all services we run; imported in appropriate host config
 
 ## working with the haccfiles
 
@@ -20,6 +22,19 @@ nix build -f . deploy.$hostname && ./result switch
 
 `$hostname` can be replaced with any hostname or group
 
+## I don't want to build this long dependency / want a cached version!
+
+If it's still available on parsons from a previous deploy, do:
+
+```shell
+nix copy --from ssh://parsons /nix/store/...
+```
+
+Note: don't just copy the .drv file (which Nix complains about if it can't
+build something), that's just the description of how to build it! If you
+don't know the actual outpath, look in the .drv file (should start with
+`Derive([("out","[the path you want]"...`)
+
 ## committing to haccfiles
 - Golden Rule: DO NOT COMMIT TO MAIN
   - exceptions apply, if you are not sure where to commit, don't commit to main