Readme: Add Github Actions nightly workflow status badge

Author: Ayesh

README.md

@@ -14,6 +14,7 @@ web development. Fast, flexible and pragmatic, PHP powers everything from your
 blog to the most popular websites in the world. PHP is distributed under the
 [PHP License v3.01](LICENSE).
 
+[![Nightly](https://github.com/php/php-src/actions/workflows/nightly.yml/badge.svg)](https://github.com/php/php-src/actions/workflows/nightly.yml)
 [![Build status](https://travis-ci.com/php/php-src.svg?branch=master)](https://travis-ci.com/github/php/php-src)
 [![Build status](https://ci.appveyor.com/api/projects/status/meyur6fviaxgdwdy/branch/master?svg=true)](https://ci.appveyor.com/project/php/php-src)
 [![Build Status](https://dev.azure.com/phpazuredevops/php/_apis/build/status/php.php-src?branchName=master)](https://dev.azure.com/phpazuredevops/php/_build/latest?definitionId=1&branchName=master)