Add library.properties for Arduino library manager.

library.properties

@@ -0,0 +1,9 @@
+name=Adafruit GFX Library
+version=1.0.0
+author=Adafruit
+maintainer=Adafruit <info@adafruit.com>
+sentence=Adafruit GFX graphics core library, this is the 'core' class that all our other graphics libraries derive from.
+paragraph=Install this library in addition to the display library for your hardware.
+category=Display
+url=https://github.com/adafruit/Adafruit-GFX-Library
+architectures=*