missing a step in http://freecompilercamp.org/new-directive-llvm

[chunhualiao]

Similar to issue 26, we may want to add a step about how to decide where to put the new directive class into the existing class hierachy, by looking at the inheritance diagram of clang::Stmt:

https://clang.llvm.org/doxygen/classclang_1_1Stmt.html