meta: update README nix installation section

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: aster-void

.gitignore

@@ -13,3 +13,7 @@ devenv.local.yaml
 # Added by cargo
 
 /target
+
+# nix
+result
+result-*

README.md

@@ -16,13 +16,15 @@ cargo install --path .
 # Run directly
 nix run github:aster-void/rollcron -- /path/to/repo
 
-# Install to profile
+# Install
 nix profile install github:aster-void/rollcron
 
-# Development shell
-nix develop
+# Open a shell
+nix shell github:aster-void/rollcron
 ```
 
+I recommend installing this via https://github.com/aster-void/nix-repository for build cache.
+
 ## Quick Start
 
 1. Create `rollcron.yaml` in your repository: