add preferred assignment syntax for zero initialization

docs/contributing.markdown

@@ -109,6 +109,13 @@ If you see bits of code around that do not follow these please don't hesitate to
 
 #### Shared pointers should be created with [boost::make_shared](http://www.boost.org/doc/libs/1_47_0/libs/smart_ptr/make_shared.html) where possible
 
+#### Use assignment operator for zero initialized numbers
+
+    double num = 0; // please
+
+    double num(0); // no
+
+
 #### Function definitions should not be separated from their arguments:
 
     void foo(int a) // please