Add GitHub Actions badges (#2571)

README.md

-# dlib C++ library [![Travis Status](https://travis-ci.com/davisking/dlib.svg?branch=master)](https://app.travis-ci.com/github/davisking/dlib)
+# dlib C++ library [![Travis Status](https://travis-ci.com/davisking/dlib.svg?branch=master)](https://app.travis-ci.com/github/davisking/dlib) [![GitHub Actions C++ Status](https://github.com/davisking/dlib/actions/workflows/build_cpp.yml/badge.svg)](https://github.com/davisking/dlib/actions/workflows/build_cpp.yml) [![GitHub Actions Python Status](https://github.com/davisking/dlib/actions/workflows/build_python.yml/badge.svg)](https://github.com/davisking/dlib/actions/workflows/build_python.yml)
 
 Dlib is a modern C++ toolkit containing machine learning algorithms and tools for creating complex software in C++ to solve real world problems. See [http://dlib.net](http://dlib.net) for the main project documentation and API reference.