add: nix instructions

Author: lessuselesss

docs/docs/guides/local-development.md

@@ -8,8 +8,9 @@ This guide covers setting up and working with Eliza in a development environment
 
 ## Prerequisites
 
-Before you begin, ensure you have:
+Before you begin, ensure you have either:
 
+### Option 1: Manual Setup
 ```bash
 # Required
 Node.js 23+
@@ -22,10 +23,17 @@ Docker (for database development)
 CUDA Toolkit (for GPU acceleration)
 ```
 
+### Option 2: Nix Development Environment
+```bash
+# Required
+Nix Package Manager
+```
+
 ## Initial Setup
 
-### 1. Repository Setup
+### Option 1: Manual Setup
 
+1. Repository Setup
 ```bash
 # Clone the repository
 git clone https://github.com/ai16z/eliza.git