Added setting the values of setters in the headers

Author: tvami

src/developing/coding-rules.md

@@ -11,7 +11,8 @@ When developing code in ldmx-sw please follow the following best practises and s
 6. Use `lowercase` for namespaces
 7. Run `just format-cpp` so the automated formatting is applied
 8. For setters, include the word `set` in the beginning of the function, e.g. `setHitValsX()`
-9. Do not use `__` in any c++ variable name
+9. Set the value of the setter in the header of the file
+10. Do not use `__` in any c++ variable name
 
 ## II. Packaging Rules
 1. Put the header files with the extension of `.h` into `Package/include/Package/MyFile.h`