Add getting started guide for Windows and macOS

Covers Python, Arduino CLI, AVR core, FastLED/OneButton libraries, make,
and port detection — with platform-specific install paths and a make-free
fallback for Windows.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -72,6 +72,8 @@ A 60-LED strip at full white = **3.6A** — far more than the Arduino's 5V pin c
 
 ## Quick start
 
+> **New to this?** See [GETTING_STARTED.md](GETTING_STARTED.md) for step-by-step instructions on installing Python, Arduino CLI, and all dependencies on Windows or macOS.
+
 ### 1. Install required libraries
 
 Arduino IDE: **Sketch → Include Library → Manage Libraries**, then install both:
@@ -187,6 +189,7 @@ Amirine_Cosplay_Lights/
 │   └── shows/                  — .txt show files (edit these)
 ├── Makefile                    — build, upload, and show conversion targets
 ├── README.md                   — this file
+├── GETTING_STARTED.md          — first-time setup for Windows and macOS
 ├── DETAILS.md                  — architecture and modification guide
 ├── PLAN.md                     — original project brief
 └── LICENSE                     — BSD 2-Clause