Add a coding style section for Exceptions (#123)

docs/Coding style.md

@@ -154,6 +154,8 @@ C++ code
      -   If required by a platform (e.g, Microsoft COM), use the appropriate
          ref-counted type, but do not expose those types to platform agnostic code, 
          prefer to use an abstraction.
+ -   Exceptions
+     -   All [user-declared destructors](https://en.cppreference.com/w/cpp/language/destructor) must be marked [noexcept](https://en.cppreference.com/w/cpp/language/noexcept_spec).
 
 Python code
 -----------