readme.md for ATmega328PB Xplained Mini. (#76)

readme.md to get users jump started.

FreeRTOS/Demo/AVR_ATmega328PB_Xplained_mini_GCC/readme.md

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+### Overview
+This directory contains a demo project for ATmega328PB Xplained Mini. 
+
+ATmega328PB has 2KB SRAM. Thus the number of demo tasks we put in this demo project is very limited. At minimum, these are included for now: 
+- register tasks to verify context switch
+- queue consumer-producer tasks to verify kernel primitives
+- an integer math task
+- a user task to blink on-board LED periodically
+- a check task to monitor if all tasks are running
+
+### Jump start
+To run the demo:
+- Install Atmel Studio IDE.
+- Open project file ```AVR_ATmega328PB_Xplained_mini_GCC.atsln```.
+- Build and debug. Could either debug with simulator or debugWIRE interface. 
+
+Note that avrdude can be used to program device as well. Though you'll need to manually add external tool, and it does not have debug capability. 
+
+### Reference
+- Board details https://www.microchip.com/DevelopmentTools/ProductDetails/atmega328pb-xmini
+- Development environment https://www.microchip.com/mplab/avr-support/atmel-studio-7