Add Arduino CLI install instructions

README.md

@@ -32,6 +32,7 @@ but felt like vital functionality was missing on the board. When designing this
 * **[How to install](#how-to-install)**
  - [Boards Manager Installation](#boards-manager-installation)
  - [Manual Installation](#manual-installation)
+ - [Arduino CLI Installation](#arduino-cli-installation)
  - [PlatformIO](#platformio)
 * **[Getting started with MightyCore](#getting-started-with-mightycore)**
 * [Wiring reference](#wiring-reference)
@@ -219,6 +220,13 @@ This installation method requires Arduino IDE version 1.8.0 or greater.
 Click on the "Download ZIP" button. Extract the ZIP file, and move the extracted folder to the location "**~/Documents/Arduino/hardware**". Create the "hardware" folder if it doesn't exist.
 Open Arduino IDE and a new category in the boards menu called "MightyCore" will show up.
 
+#### Arduino CLI Installation
+Run the following command in a terminal: 
+
+```
+arduino-cli core install MightyCore:avr --additional-urls https://mcudude.github.io/MightyCore/package_MCUdude_MightyCore_index.json
+```
+
 #### PlatformIO
 [PlatformIO](http:https://platformio.org) is an open-source ecosystem for IoT and embedded systems, and supports MightyCore.