Add a package.json file to fix platformio build

For more details see:
adafruit#571
platformio/platformio-core#3693

Author: dmpolukhin

package.json

@@ -0,0 +1,7 @@
+{
+  "description": "Adafruit Arduino Framework for NRF52",
+  "name": "framework-arduinoadafruitnrf52",
+  "system": "*",
+  "url": "https://github.com/adafruit/Adafruit_nRF52_Arduino",
+  "version": "0.21.1"
+}