add lib prop + readme

Author: karlsoderby

library.properties

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+name=Arduino_GigaDisplay_GFX
+version=1.0.0
+author=Arduino
+maintainer=Arduino <info@arduino.cc>
+sentence=GFX library for GIGA Display Shield
+paragraph=
+category=Display
+url=https://docs.arduino.cc/tutorials/giga-display-shield/gfx-guide
+architectures=mbed_giga
+includes=Arduino_GigaDisplay_GFX.h
+depends=Adafruit GFX Library

readme.md

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+# Arduino_GigaDisplay_GFX
+
+**Arduino_GigaDisplay_GFX** is a layer library for using methods from the [Adafruit GFX]() graphics core library, developed for the [Arduino GIGA Display Shield]() and the [Arduino GIGA R1 WiFi]() board.
+
+It can be used to draw anything from single pixels to advanced geometrical shapes, and supports printing text / values with configurable font sizes. 
+
+To get started with this library and the shield, visit the [GIGA Display Shield GFX Guide](https://docs.arduino.cc/tutorials/giga-display-shield/gfx-guide).
+
+>**Note:** this library requires version `4.0.6` and above of the [GIGA core](https://github.com/arduino/ArduinoCore-mbed) to be installed.