Add github actions build badge

Author: jericks

README.rst

@@ -1,6 +1,9 @@
 .. image:: https://travis-ci.org/geoscript/geoscript-groovy.svg?branch=master
     :target: https://travis-ci.org/geoscript/geoscript-groovy
 
+.. image:: https://github.com/geoscript/geoscript-groovy/workflows/Maven%20Build/badge.svg
+    :target: https://github.com/geoscript/geoscript-groovy/actions
+
 GeoScript Groovy
 ================
 GeoScript Groovy is the `Groovy <http://groovy.codehaus.org/>`_ implementation of `GeoScript <http://geoscript.org>`_.  GeoScript is a geospatial scripting API for the JVM that contains one API and four implementations (`Python <https://github.com/jdeolive/geoscript-py>`_, `JavaScript <https://github.com/tschaub/geoscript-js>`_, `Scala <https://github.com/dwins/geoscript.scala>`_, and `Groovy <https://github.com/jericks/geoscript-groovy>`_).