Add badge for GitHub Actions.

Author: ioquatix

README.md

@@ -1,5 +1,6 @@
 # OpenSSL for Ruby
 
+[![Actions Status](https://github.com/ruby/openssl/workflows/OpenSSL/badge.svg)](https://github.com/ruby/openssl/actions?workflow=OpenSSL)
 [![Build Status](https://travis-ci.org/ruby/openssl.svg?branch=master)](https://travis-ci.org/ruby/openssl)
 [![Build status](https://ci.appveyor.com/api/projects/status/b8djtmwo7l26f88y/branch/master?svg=true)](https://ci.appveyor.com/project/ruby/openssl/branch/master)