syntax: go: add author and license line

In the original submission (https://savannah.gnu.org/patch/?8189)
the submitter said: "I hereby put it into the public domain."

Public-domain license: https://directory.fsf.org/wiki/License:CC0

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>

syntax/go.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for Go.
 
+## Original author:  Robert Clausecker
+## License:  CC0 (public domain)
+
 syntax go "\.go$"
 comment "//"