Make CI badge clickable

Author: valeros

README.md

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 # Espressif 8266: development platform for [PlatformIO](http://platformio.org)
 
-![alt text](https://github.com/platformio/platform-espressif8266/workflows/Examples/badge.svg "Espressif 8266 development platform")
+[![Build Status](https://github.com/platformio/platform-espressif8266/workflows/Examples/badge.svg)](https://github.com/platformio/platform-espressif8266/actions)
 
 Espressif Systems is a privately held fabless semiconductor company. They provide wireless communications and Wi-Fi chips which are widely used in mobile devices and the Internet of Things applications.