Create contributing information for github.

.github/CONTRIBUTING.md

+This project is now tracked on our hosted gitlab server at:
+> https://git.duniter.org/clients/cesium/cesium
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github would generate more work for the main contributors.
+
+Issues can be submitted on github. However, all issues created on github will be duplicated on gitlab manually and closed with a link to the gitlab issue.