docs: write docs, add README

README.md

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+# Rensa
+
+Rensa is a powerful and flexible Nix flake framework designed to organize your Nix projects into logical units called "Cells".
+It is heavily inspired by [divnix/std](https://github.com/divnix/std) but aims to provide a streamlined experience.
+Feel free to check out std's awesome [docs](https://std.divnix.com/reference/std.html) too.
+
+## Features
+
+- **Cells**: Organize your code into functional domains (e.g., `backend`, `frontend`, `devops`).
+- **Blocks**: Define types of outputs within a cell (e.g., `packages`, `devShells`, `nixosConfigurations`).
+- **Actions**: Integrate with the CLI to run commands defined in your cells.
+- **Cell Flakes**: Each cell can be its own flake, allowing for modular dependency management.
+
+## Quick Start
+
+1. **Initialize a Flake**: Create a `flake.nix` that uses `rensa`.
+1. **Define Cells**: Create a `cells/` directory.
+1. **Add Blocks**: Add `.nix` files in your cells to define outputs.
+
+### Example `flake.nix`
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    ren.url = "gitlab:rensa-nix/core";
+  };
+
+  outputs = {ren, ...}@inputs: ren.buildWith {
+    inherit inputs;
+    cellsFrom = ./cells;
+    cellBlocks = with ren.blocks; [
+      (simple "packages")
+      (simple "devShells")
+    ];
+  } {
+    packages = ren.select inputs.self [
+      ["my-cell" "packages"]
+    ];
+  };
+}
+```
+
+See the [docs](https://rensa.projects.tf) for more.