Document class file name rule

This rule was followed in practice, but not documented so far.

development/coding/Rules

@@ -290,6 +290,10 @@ Formatting
   
   - private/protected functions are also camel-case
   
+  - Each class is implemented in a separate pair of .h/.cpp files named like
+    the class. Exceptions are tiny helper classes which are closely tied to
+    the main class.
+
 
   New types are capitalized, so this goes for typedefs, classes, structs
   and enums.