Add contribution guide

CONTRIBUTING.md

+To contribute to this project, please just create an account here:  
+https://gitlab.kg6wxc.net  
+
+Create your own fork if you like and submit merge requests,  
+or just use the issues area to suggest new ideas.  
+GitLab is basically just like GitHub,  
+if you know how to use that you should be fine.  
+
+kg6wxc retains the right to approve new code to the "main" branch.
+Anyone is free to contribute.  
+Go ahead and create new fork with additial options/settings/maps, etc...  
+
+Fork this off to github if you like even, it is here for everyone!