add github messages

.github/CONTRIBUTING.md

+This project is now tracked on our hosted gitlab server at:
+https://git.duniter.org/nodes/rust/duniter-rs
+
+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 must be submitted on gitlab : https://git.duniter.org/nodes/rust/duniter-rs/issues
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.github/ISSUE_TEMPLATE.md

+Please note that all issues must be opened at https://git.duniter.org/nodes/rust/duniter-rs/issues/new?issue
+
+Thank you for your understanding.
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.github/PULL_REQUEST_TEMPLATE.md

+This project is now tracked on our hosted gitlab server at:
+https://git.duniter.org/nodes/rust/duniter-rs
+
+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, therefore we would really appreciate if you can create instead merge requests on our hosted gitlab instance.
+
+Thank you for your understanding.
\ No newline at end of file