Added instructions for installation with boards manager (stolen directly from esp8266)

lbernstone · lbernstone · commit 003b64c19c11 · 2018-07-08T10:37:44.000-06:00
diff --git a/docs/arduino-ide/boards_manager.md b/docs/arduino-ide/boards_manager.md
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+Installation instructions using Arduino IDE Boards Manager
+==========================================================
+
+Starting with 1.6.4, Arduino allows installation of third-party platform packages using Boards Manager. We have packages available for Windows, Mac OS, and Linux (32 and 64 bit).
+
+- 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).
+- Start Arduino and open Preferences window.
+- Enter ```https://dl.espressif.com/dl/package_esp32_dev_index.json``` into *Additional Board Manager URLs* field. You can add multiple URLs, separating them with commas.
+- Open Boards Manager from Tools > Board menu and install *esp32* platform (and don't forget to select your ESP32 board from Tools > Board menu after installation).
+
+#### Latest release [![Latest release](https://img.shields.io/github/release/espressif/arduino-esp32.svg)](https://github.com/espressif/arduino-esp32/releases/latest/)
+Boards manager link: `https://dl.espressif.com/dl/package_esp32_dev_index.json`
