Update README.md

Author: fpistm

README.md

@@ -1,5 +1,6 @@
 # Arduino core support for STM32 based boards
 [![GitHub release](https://img.shields.io/github/release/stm32duino/Arduino_Core_STM32.svg)](https://github.com/stm32duino/Arduino_Core_STM32/releases/latest)
+![GitHub All Releases](https://img.shields.io/github/downloads/stm32duino/Arduino_Core_STM32/total.svg?label=downloads%20since%201.4.0) 
 [![GitHub commits](https://img.shields.io/github/commits-since/stm32duino/Arduino_Core_STM32/1.6.0.svg)](https://github.com/stm32duino/Arduino_Core_STM32/compare/1.6.0...master)
 [![Build Status](https://travis-ci.com/stm32duino/Arduino_Core_STM32.svg?branch=master)](https://travis-ci.com/stm32duino/Arduino_Core_STM32)