readme update

Author: hathach

README.md

@@ -2,16 +2,44 @@
 
 [![Build Status](https://github.com/adafruit/Adafruit_TinyUSB_Arduino/workflows/Build/badge.svg)](https://github.com/adafruit/Adafruit_TinyUSB_Arduino/actions) [![License](https://img.shields.io/badge/license-MIT-brightgreen.svg)](https://opensource.org/licenses/MIT)
 
-This library works with platform that includes [TinyUSB stack](https://github.com/hathach/tinyusb), typically you will see **Adafruit_TinyUSB_Core** inside core folder. Supported platform are:
+This library is a Arduino-friendly version of [TinyUSB](https://github.com/hathach/tinyusb) stack. It is designed with structure and APIs that are easily integrated to existing or new Arduino Core. Supported platform including: 
 
 - [Adafruit_nRF52_Arduino](https://github.com/adafruit/Adafruit_nRF52_Arduino)
 - [Adafruit ArduinoCore-samd](https://github.com/adafruit/ArduinoCore-samd) **TinyUSB** must be selected in menu `Tools->USB Stack`
 
-In addition to CDC that provided by the platform core, this library provide platform-independent for
+Current class drivers supported are
 
+- Communication (CDC): which is used to implement `Serial` monitor
 - Human Interface Device (HID): Generic (In & Out), Keyboard, Mouse, Gamepad etc ...
 - Mass Storage Class (MSC): with multiple LUNs
 - Musical Instrument Digital Interface (MIDI)
 - WebUSB with vendor specific class
 
+For supported ArduinoCore, to use this library, you only need to have `<Adafruit_TinyUSB.h>` in your sketch. If your ArduinoCore does not support TinyUSB library yet, it is rather simple to port.
+
+## Class Driver API
+
 More document to write ... 
+
+## Porting Guide
+
+It is rather easy if you want to integrate TinyUSB lib to your ArduinoCore.
+
+### ArduinoCore Changes
+
+1. Add this repo as submodule (or have local copy) at your ArduioCore/libraries/Adafruit_TinyUSB_Arduino (much like SPI).
+2. Since Serial as CDC is considered as part of the core, we need to have `#include "Adafruit_USBD_CDC.h"` within your `Arduino.h`. For this to work, your `platform.txt` include path need to have `"-I{runtime.platform.path}/libraries/Adafruit_TinyUSB_Arduino/src/arduino"`.
+3. In your `main.cpp` before setup() invoke the `TinyUSB_Device_Init(rhport)`. This will initialize usb device hardware and tinyusb stack and also include Serial as an instance of CDC class.
+4. `TinyUSB_Device_Task()` must be called whenever there is new USB event. Depending on your core and MCU with or without RTOS. There are many ways to run the task. For example:
+  - Use USB IRQn to set flag then invoke function later on after exiting IRQ.
+  - Just invoke function after the loop(), within yield(), and delay()
+5. `TinyUSB_Device_FlushCDC()` should also be called often to send out Serial data as well.
+6. Note: For low power platform that make use of WFI()/WFE(), extra care is required before mcu go into low power mode. Check out my PR to circuipython for reference https://github.com/adafruit/circuitpython/pull/2956
+
+### Library Changes
+
+In addition to core changes, library need to be ported to your platform. Don't worry, tinyusb stack has already done most of heavy-lifting. You only need to write a few APIs
+
+1. `TinyUSB_Port_InitDeviceController()` hardware specific (clock, phy) to enable usb hardware. This API is called as part of TinyUSB_Device_Init() invocation.
+2. `TinyUSB_Port_EnterDFU()` which is called when device need to enter DFU mode, usually by touch1200 feature
+3. `TinyUSB_Port_GetSerialNumber()` which is called to get unique MCU Serial ID to used as Serial string descriptor.