add Github actions badge

Author: Vladimir Kotal

README.md

@@ -3,6 +3,7 @@ Copyright (c) 2006, 2020 Oracle and/or its affiliates. All rights reserved.
 
 
 # OpenGrok - a wicked fast source browser
+![Github actions build](https://github.com/oracle/opengrok/workflows/Build/badge.svg?branch=master)
 [![Travis status](https://travis-ci.org/oracle/opengrok.svg?branch=master)](https://travis-ci.org/oracle/opengrok)
 [![Coverage status](https://coveralls.io/repos/oracle/opengrok/badge.svg?branch=master)](https://coveralls.io/r/oracle/opengrok?branch=master)
 [![SonarQube status](https://sonarcloud.io/api/project_badges/measure?project=org.opengrok%3Aopengrok-top&metric=alert_status)](https://sonarcloud.io/dashboard?id=org.opengrok%3Aopengrok-top)