Add link to translation.md

Author: chennes

technical/index.md

@@ -11,6 +11,7 @@ Technical information of interest to Contributors.
 - [Source Tree Basics](./SourceTreeBasics.md)
 - [The Application Module](./TheApplicationModule.md)
 - [The Source Code in More Depth](https://wiki.freecad.org/The_FreeCAD_source_code)
+- [Writing Code for Translation](./translation.md)
 - The FreeCAD developer's tool set:
     - C++ and Python
     - [Qt](https://www.qt.io/): a cross platform development framework