Add github actions badge to README

This replaces the old Travis CI badge that didn't mean anything.

README.md

-# yaml-cpp [![Build Status](https://travis-ci.org/jbeder/yaml-cpp.svg?branch=master)](https://travis-ci.org/jbeder/yaml-cpp) [![Documentation](https://codedocs.xyz/jbeder/yaml-cpp.svg)](https://codedocs.xyz/jbeder/yaml-cpp/)
+# yaml-cpp ![Build Status](https://github.com/jbeder/yaml-=cpp/actions/workflows/build.yaml/badge.svg) [![Documentation](https://codedocs.xyz/jbeder/yaml-cpp.svg)](https://codedocs.xyz/jbeder/yaml-cpp/)
 
 `yaml-cpp` is a [YAML](http://www.yaml.org/) parser and emitter in C++ matching the [YAML 1.2 spec](http://www.yaml.org/spec/1.2/spec.html).