Add Gitter badge (espressif#96)

Author: gitter-badger

README.md

@@ -1,5 +1,7 @@
 # Arduino core for ESP32 WiFi chip
 
+[![Join the chat at https://gitter.im/espressif/arduino-esp32](https://badges.gitter.im/espressif/arduino-esp32.svg)](https://gitter.im/espressif/arduino-esp32?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 - [Development Status](#development-status)
 - Installing options:
   + [Using Arduino IDE](#using-arduino-ide)