syntax: markdown: add author and license line

Signed-off-by: Ryan Westlund <rlwestlund@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>

syntax/markdown.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for Markdown files.
 
+## Original authors:  Ryan Westlund and Benno Schulenberg
+## License:  GPL version 3 or newer
+
 syntax markdown "\.md$"
 comment "<!--|-->"