Added a new rule about class inheritance and interfaces

Author: javiereguiluz

contributing/code/standards.rst

@@ -153,6 +153,9 @@ Structure
   that are not intended to be instantiated from the outside and thus are not
   concerned by the `PSR-0`_ and `PSR-4`_ autoload standards;
 
+* Declare the class inheritance (if any) and all the implemented interfaces (if
+  any) in the same line as the class name;
+
 * Declare class properties before methods;
 
 * Declare public methods first, then protected ones and finally private ones.