docs: write docs, add README

docs/libraries.md

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+# Related Libraries
+
+Rensa has a growing ecosystem of compatible libraries designed to work seamlessly with its cell structure.
+These libraries typically expose a `lib` output that accepts `pkgs` as an argument.
+
+## Available Libraries
+
+Here are some common libraries you might want to use:
+
+- [**Rensa Devshell**](https://devshell.rensa.projects.tf): _Minimal devshell implementation using Modules._
+- [**Rensa Devtools**](https://devtools.rensa.projects.tf): _Utils and dev tools for the Rensa ecosystem._
+- [**Rensa Utils**](https://utils.rensa.projects.tf): _Utilities for NixOS/darwin systems, home & disko configurations etc._
+- [**Nix-GitLab-CI**](https://nix-gitlab-ci.projects.tf): _Allows (advanced) configuration of GitLab CI using Nix._
+- [**Nixible**](https://nixible.projects.tf): _Ansible but with Nix._
+- [**Tofunix**](https://tofunix.projects.tf): _Combining Nix and Terraform for reproducibility and developer experience._
+- [**Nixlets**](https://nixlets.projects.tf): _Nixlets - like Helm Charts or Grafana Tanka but instead using Nix._
+- [**Nixtest**](https://nixtest.projects.tf): _Test runner for Nix code._
+- [**Soonix**](https://soonix.projects.tf): _Auto generated project files from Nix code, with gitignore handling and many generators._
+- [**NixMkDocs**](https://nix-mkdocs.projects.tf): _Nix library for easy mkdocs integration into projects._
+- [**Torikae**](https://torikae.projects.tf): _Simple CLI to replace versions in files._
+
+## Usage Pattern
+
+When using these libraries within a Cell Flake, you can instantiate them using the `pkgs` passed down from the parent flake.
+This follows the "Single Instantiation" best practice.
+
+### Example: `cells/mycell/flake.nix`
+
+```nix
+{
+  inputs = {
+    # Add the library to your inputs
+    devshell.url = "gitlab:rensa-nix/devshell?dir=lib";
+    
+    # ... other inputs
+  };
+
+  outputs = i:
+    i
+    // {
+      # Instantiate the library using the parent's pkgs
+      dslib = i.devshell.lib { inherit (i.parent) pkgs; };
+    };
+}
+```
+
+Now you can use `inputs.dslib` in your blocks!