add readme

README.md

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+# NixOS & Home Manager config
+
+## Structure
+
+- **home**: Home Manager modules
+- **hosts**: Host-specific configs
+- **infra**: Terraform/OpenTofu files
+- **secrets**: Age-encrypted secrets shared between multiple hosts.
+  Host-specific secrets are stored in their own directories.
+- **system**: Base NixOS modules shared among all hosts