message: Update coding guidelines (use pragma once)
diff: @@ -71,8 +71,7 @@ Example for the Impatient
  ```cpp
  // libcaf_example/caf/example/my_class.hpp
  -#ifndef CAF_EXAMPLE_MY_CLASS_HPP
  -#define CAF_EXAMPLE_MY_CLASS_HPP
  +#pragma once
  #include <string>
  @@ -118,8 +117,6 @@ private:
  } // namespace example
  } // namespace caf
  -
  -#endif // CAF_EXAMPLE_MY_CLASS_HPP
  ```
  ```cpp
  @@ -329,8 +326,7 @@ Headers
  is located at `libcaf_example/caf/example/my_class.hpp` and the
  source file at `libcaf_example/src/my_class.cpp`.
  -- All header files should use `#define` guards to prevent multiple inclusion.
  - The symbol name is `<RELATIVE>_<PATH>_<TO>_<FILE>_HPP`.
  +- All header files use `#pragma once` to prevent multiple inclusion.
  - Do not `#include` when a forward declaration suffices.
  