Add github action status badge

Author: jericks

README.rst

@@ -1,6 +1,9 @@
 .. image:: https://travis-ci.org/jericks/geo-shell.svg?branch=master
     :target: https://travis-ci.org/jericks/geo-shell
 
+.. image:: https://github.com/jericks/geo-shell/workflows/Maven%20Build/badge.svg
+    :target: https://github.com/jericks/geo-shell/actions
+
 geo-shell
 ---------
 geo-shell is an interactive shell for geospatial analysis.