Added 'Travis-CI' badge status to 'Readme'.

Author: MrPointer

README.md

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 # Arduino-CMake NG
 
 [![AppVeyor Build](https://ci.appveyor.com/api/projects/status/github/arduino-cmake/Arduino-CMake-NG?svg=true&passingText=Windows%20-%20Passing&failingText=Windows%20-%20Failing&pendingText=Windows%20-%20Pending)](https://ci.appveyor.com/project/arduino-cmake/arduino-cmake-ng)
+[![Travis CI](https://img.shields.io/travis/arduino-cmake/Arduino-CMake-NG/branch=develop.svg?label=Linux and OS X &logo=travis)](https://travis-ci.org/arduino-cmake/Arduino-CMake-NG)
 
 **Arduino-CMake** is a framework which allows developers to write Arduino-based programs using any tool that supports cmake. *Arduino-based*? There are many other frameworks out there built upon Arduino's base, such as ESP32, and **we support that**.
 In other words, developers can use their favorite IDEs or text editors on their favorite OS to develop Arduino programs!