Add GitHub Actions badges. (#1172)

Author: junaruga

README.md

@@ -1,7 +1,12 @@
 # Mysql2 - A modern, simple and very fast MySQL library for Ruby - binding to libmysql
 
-Travis CI [![Travis CI Status](https://travis-ci.org/brianmario/mysql2.png)](https://travis-ci.org/brianmario/mysql2)
-Appveyor CI [![Appveyor CI Status](https://ci.appveyor.com/api/projects/status/github/sodabrew/mysql2)](https://ci.appveyor.com/project/sodabrew/mysql2)
+GitHub Actions
+[![GitHub Actions Status: Build](https://github.com/brianmario/mysql2/actions/workflows/build.yml/badge.svg)](https://github.com/brianmario/mysql2/actions/workflows/build.yml)
+[![GitHub Actions Status: Container](https://github.com/brianmario/mysql2/actions/workflows/container.yml/badge.svg)](https://github.com/brianmario/mysql2/actions/workflows/container.yml)
+Travis CI
+[![Travis CI Status](https://travis-ci.org/brianmario/mysql2.png)](https://travis-ci.org/brianmario/mysql2)
+Appveyor CI
+[![Appveyor CI Status](https://ci.appveyor.com/api/projects/status/github/sodabrew/mysql2)](https://ci.appveyor.com/project/sodabrew/mysql2)
 
 The Mysql2 gem is meant to serve the extremely common use-case of connecting, querying and iterating on results.
 Some database libraries out there serve as direct 1:1 mappings of the already complex C APIs available.