doc: Add redirect for GUI issues and pull requests

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,6 +3,10 @@
 
 Pull requests without a rationale and clear improvement may be closed
 immediately.
+
+GUI-related pull requests should be opened against
+https://github.com/bitcoin-core/gui
+first. See CONTRIBUTING.md
 -->
 
 <!--