Add T.A.M.I Drawbot modifications changelog to README

- Added changelog section documenting all modifications since forking
- Documented December 4, 2025 changes (inverted servo PWM, M5 fix)
- Listed earlier modifications (servo mode, homing fix)

PLATFORMIO.md

@@ -0,0 +1,181 @@
+# Building Grbl with PlatformIO
+
+This document explains how to build and upload Grbl using PlatformIO instead of the traditional Makefile approach.
+
+## Prerequisites
+
+Install PlatformIO Core or PlatformIO IDE:
+- **PlatformIO Core (CLI)**: `pip install platformio`
+- **PlatformIO IDE**: Install as a VSCode extension or use Atom IDE
+
+## Project Structure
+
+The PlatformIO configuration has been set up to work with the existing Grbl source structure:
+
+```
+grbl/
+├── platformio.ini          # PlatformIO configuration
+├── src/
+│   └── main.cpp           # Arduino framework wrapper
+├── grbl/                  # Grbl source code (unchanged)
+│   ├── *.c, *.h          # Core Grbl files
+│   ├── config.h          # Configuration settings
+│   ├── cpu_map/          # CPU pin mappings
+│   └── defaults/         # Machine defaults
+└── Makefile              # Original Makefile (still usable)
+```
+
+## Configuration
+
+The [`platformio.ini`](platformio.ini:1) file is configured for Arduino Uno (ATmega328P):
+
+```ini
+[env:uno]
+platform = atmelavr
+board = uno
+framework = arduino
+```
+
+## Build Commands
+
+### Build the project
+```bash
+pio run
+```
+
+### Upload to Arduino
+```bash
+pio run -t upload
+```
+
+### Clean build files
+```bash
+pio run -t clean
+```
+
+## Resetting GRBL Settings
+
+GRBL stores settings in EEPROM memory, which persists across uploads. When you upload new firmware, your previous settings remain. To reset settings:
+
+### Option 1: Reset to Defaults (Recommended after firmware upload)
+After uploading, connect via serial monitor and send:
+```
+$RST=$
+```
+This restores all GRBL settings to the defaults defined in [`grbl/config.h`](grbl/config.h:1).
+
+### Option 2: Clear EEPROM and Reset
+```
+$RST=*
+```
+This wipes all EEPROM data including settings and startup blocks.
+
+### Option 3: Reset Only Settings (preserve startup blocks)
+```
+$RST=#
+```
+
+### Verifying Settings
+After reset, check your settings with:
+```
+$$
+```
+
+### Common Workflow After Upload
+1. Upload firmware: `pio run -t upload`
+2. Open serial monitor: `pio run -t monitor`
+3. Send reset command: `$RST=$`
+4. Verify settings: `$$`
+5. Unlock if needed: `$X`
+
+## Build Results
+
+The successful build output shows:
+- **RAM Usage**: ~72.5% (1484 bytes / 2048 bytes)
+- **Flash Usage**: ~91.6% (29544 bytes / 32256 bytes)
+
+Compiled firmware is located at `.pio/build/uno/firmware.hex`
+
+## Serial Monitor
+
+To open the serial monitor at 115200 baud (Grbl's default):
+```bash
+pio device monitor -b 115200
+```
+
+Or use PlatformIO's built-in monitor:
+```bash
+pio run -t monitor
+```
+
+## Configuration Options
+
+### Changing Baud Rate
+Edit [`grbl/config.h`](grbl/config.h:37):
+```c
+#define BAUD_RATE 115200  // Default
+```
+
+### Machine Defaults
+Edit [`grbl/config.h`](grbl/config.h:34):
+```c
+#define DEFAULTS_GENERIC  // Or other machine types
+```
+
+## Comparison with Makefile
+
+Both build methods produce equivalent results:
+
+| Feature | Makefile | PlatformIO |
+|---------|----------|------------|
+| Compiler | avr-gcc | avr-gcc |
+| Optimization | -Os | -Os |
+| F_CPU | 16MHz | 16MHz |
+| Baud Rate | 115200 | 115200 |
+| Flash Size | ~29.5KB | ~29.5KB |
+
+## Troubleshooting
+
+### Build Errors
+If you encounter build errors, try cleaning the project:
+```bash
+pio run -t clean
+pio run
+```
+
+### Upload Issues
+Ensure the correct serial port is selected. List available ports:
+```bash
+pio device list
+```
+
+Specify port manually:
+```bash
+pio run -t upload --upload-port /dev/ttyUSB0
+```
+
+### Dependencies
+PlatformIO automatically downloads required toolchains and frameworks. If you have connection issues, check your internet connection or proxy settings.
+
+## Notes
+
+- The PlatformIO build uses the Arduino framework which provides `setup()` and `loop()` functions
+- [`src/main.cpp`](src/main.cpp:1) wraps Grbl's native [`main()`](grbl/main.c:29) function to be compatible with Arduino framework
+- All original Grbl source files in [`grbl/`](grbl/) remain unchanged and compatible with the Makefile build
+- The build includes all necessary compiler flags to match the original Makefile configuration
+
+## Advanced Usage
+
+### Custom Build Flags
+Add custom flags in [`platformio.ini`](platformio.ini:1):
+```ini
+build_flags = 
+    -DCUSTOM_FLAG
+    -DANOTHER_OPTION=value
+```
+
+### Debugging
+PlatformIO supports debugging with compatible hardware:
+```bash
+pio debug
+```