Update README.md (#4730)

Clarify the folder of the git installation method for Mac

README.md

@@ -33,8 +33,12 @@ Documentation: [https://arduino-esp8266.readthedocs.io/en/2.4.1/](https://arduin
 ### Using git version
 [![Linux build status](https://travis-ci.org/esp8266/Arduino.svg)](https://travis-ci.org/esp8266/Arduino)
 
-- Install Arduino 1.8.2 from the [Arduino website](http://www.arduino.cc/en/main/software).
-- Go to Arduino directory
+- Install the current upstream Arduino IDE at the 1.8 level or later. The current version is at the [Arduino website](http://www.arduino.cc/en/main/software).
+- Go to Arduino application directory
+  - For Mac OS X normally: 
+  ```bash
+  cd /Applications/Arduino.app/Contents/Java
+  ```
 - Clone this repository into hardware/esp8266com/esp8266 directory (or clone it elsewhere and create a symlink)
 ```bash
 cd hardware