Added section about ArduinoJson dependency

Author: kotl

Diff for: README.md

@@ -19,6 +19,9 @@ The Arduino library is [under heavy development](https://github.com/googlesample
 
 - [FirebaseArduino API Reference](http://firebase-arduino.readthedocs.io/)
 
+## Dependencies
+- FirebaseArduino now depends on [ArduinoJson library](https://github.com/bblanchon/ArduinoJson) instead of containing it's own version of it. Please either use Library Manager or download specific version of the library from github.
+
 ## Disclaimer
 
 *This is not an official Google product*.