add a readme w/ related license

README.md

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+# Zephyr UART + LED - Nucleo G474RE
+
+A learning project for [Zephyr RTOS](https://zephyrproject.org/) targeting the **STM32 Nucleo G474RE** dev board! I have always seen Zephyr projects in the wild and want to document my learning process. I've used FreeRTOS in the past and want to rewrite some projects using Zephyr 🙂
+
+----
+
+## Hardware
+
+| | |
+|---|---|
+| **Board** | ST Nucleo G474RE |
+| **MCU** | STM32G474RE (ARM Cortex-M4F, 170 MHz) |
+| **UART** | LPUART1 via onboard ST-Link VCP (PA2/PA3) |
+| **LED** | LD2 on PA5 |
+
+Connect via USB to the Nucleo's ST-Link port
+
+----
+
+## Features
+
+- **Interrupt-driven UART RX** - ISR writes bytes into a ring buffer
+- **Dedicated RX thread** - sleeps on a semaphore, wakes on data arrival
+- **Line accumulation** - buffers input until `\r` or `\n`
+- **Formatted output** - prints received line with length and uptime timestamp
+- **LED** - on PA5 configured via devicetree overlay (for testing)
+
+----
+
+## Getting Started
+
+### Prerequisites
+
+- [Zephyr SDK](https://docs.zephyrproject.org/latest/develop/getting_started/index.html) installed
+- `west` installed and workspace initialized
+
+### Build & Flash
+
+```bash
+west build          # board is set in CMakeLists.txt
+west flash
+
+(optionally)
+west debug
+```
+
+### Serial Monitor
+
+Connect a terminal to the ST-Link Virtual COM Port at a baud rate of **115200**:
+
+Personally, I just use PuTTY!
+
+Type a line and press Enter:
+
+```
+─────────────────────────────
+│ RX: hello
+│ Len: 5,   Time: 6741
+─────────────────────────────
+```
+
+---
+
+## License
+
+[Apache 2.0](https://www.apache.org/licenses/LICENSE-2.0)