add build status badge for actions

Author: mrtazz

README.md

@@ -1,5 +1,5 @@
 # REST client for C++
-[![Build Status](https://travis-ci.org/mrtazz/restclient-cpp.svg?branch=master)](https://travis-ci.org/mrtazz/restclient-cpp)
+![build status](https://github.com/mrtazz/restclient-cpp/actions/workflows/tests.yaml/badge.svg)
 [![Coverage Status](https://coveralls.io/repos/mrtazz/restclient-cpp/badge.svg?branch=master&service=github)](https://coveralls.io/github/mrtazz/restclient-cpp?branch=master)
 [![Packagecloud](https://img.shields.io/badge/packagecloud-available-brightgreen.svg)](https://packagecloud.io/mrtazz/restclient-cpp)
 [![doxygen](https://img.shields.io/badge/doxygen-reference-blue.svg)](http://code.mrtazz.com/restclient-cpp/ref/)