usb-device-mtp: Add documentation for MTP driver

- Create detailed README.md for usb-device-mtp package
- Update main USB README.md to include MTP driver information
- Document usage, configuration options, and limitations

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: andrewleech

micropython/usb/README.md

@@ -135,6 +135,16 @@ USB MIDI devices in MicroPython.
 The example [midi_example.py](examples/device/midi_example.py) demonstrates how
 to create a simple MIDI device to send MIDI data to and from the USB host.
 
+### Package usb-device-mtp
+
+This package provides the `usb.device.mtp` module, which implements the Media Transfer
+Protocol (MTP) to expose MicroPython's filesystem to host computers.
+
+The example [mtp_example.py](examples/device/mtp_example.py) demonstrates how to
+create an MTP device interface that allows browsing and transferring files to/from 
+the MicroPython filesystem using standard file managers on Windows, macOS, and Linux
+without requiring special drivers.
+
 ### Limitations
 
 #### Buffer thread safety

micropython/usb/usb-device-mtp/README.md

@@ -0,0 +1,87 @@
+# USB MTP Device Driver for MicroPython
+
+This package provides a USB Media Transfer Protocol (MTP) interface for MicroPython, enabling filesystem access from host computers via a standard protocol.
+
+## Features
+
+- Implements the Media Transfer Protocol (MTP) based on the PTP/PIMA 15740 Still Image standard
+- Compatible with Windows, macOS, and Linux hosts without requiring special drivers
+- Exposes MicroPython's filesystem to the host computer
+- Supports basic file operations:
+  - Browsing directories
+  - Uploading files to the device
+  - Downloading files from the device
+  - Deleting files and directories
+- Optional debug logging to assist with troubleshooting
+
+## Installation
+
+Install the package using MicroPython's package manager:
+
+```
+mpremote mip install usb-device-mtp
+```
+
+## Usage
+
+The basic usage pattern is:
+
+```python
+import usb.device
+from usb.device.mtp import MTPInterface
+
+# Create an MTP interface that exposes a specific path as storage
+mtp = MTPInterface(storage_path="/")
+
+# Initialize the USB device with the MTP interface
+# Keep builtin_driver=True to maintain the serial REPL alongside MTP
+usb.device.get().init(mtp, builtin_driver=True)
+
+# At this point, the MTP interface is available to the host computer
+# Your MicroPython filesystem will be accessible via the standard file manager
+```
+
+See [mtp_example.py](/examples/device/mtp_example.py) for a complete example program.
+
+## Configuration Options
+
+The `MTPInterface` constructor accepts the following parameters:
+
+- `storage_path`: Root directory to expose via MTP (default: "/")
+- `rx_size`: Size of the receive buffer in bytes (default: 4096)
+- `tx_size`: Size of the transmit buffer in bytes (default: 4096)
+- `debug`: Enable detailed debug logging (default: False)
+
+## Runtime USB Considerations
+
+When running the MTP interface:
+
+1. The device may disconnect temporarily from the host while USB reconfiguration occurs
+2. The MicroPython REPL serial interface will remain accessible if you use `builtin_driver=True`
+3. If you add the MTP initialization code to `boot.py`, remember to include `builtin_driver=True` to maintain access to the device
+
+## Limitations
+
+- Performance may vary depending on the MicroPython board's USB hardware
+- Large file transfers might be slower than with dedicated file transfer tools
+- Limited support for MTP events and advanced features
+- Only one storage (the specified path) is exposed
+
+## Implementation Details
+
+This driver implements the USB MTP protocol by extending the `Interface` class from `usb.device.core`. It uses the Still Image class (0x06) with the PIMA 15740 protocol (0x01) specification to provide compatibility with standard MTP implementations on host operating systems.
+
+The driver requires three endpoints:
+- Bulk OUT endpoint for receiving commands and data
+- Bulk IN endpoint for sending data and responses
+- Interrupt IN endpoint for sending events (currently unused)
+
+## Debugging
+
+To enable verbose debug logging:
+
+```python
+mtp = MTPInterface(storage_path="/", debug=True)
+```
+
+This will output detailed information about USB transfers, MTP commands, and filesystem operations, which can be helpful when troubleshooting connection issues.