- leave /guides/upgrade as symlink

upgrade/v1606-Developer-Upgrade-Guide.md

+<!-- --- title: Developer Upgrade Guide (OpenFOAM-v1606) -->
+
+[![home](/icons/home.svg "wiki home")](/home)
+[![upgrade](/icons/chevrons-up.svg "upgrade guide")][upgrade-guide]
+[![code](/icons/code.svg "coding patterns")][code-patterns]
+
+
+[code-patterns]: /coding/patterns/patterns
+[upgrade-guide]: /upgrade/upgrade
+
+[v1606-notes]: https://www.openfoam.com/releases/openfoam-v1606+/