improve design docs

[ArcherWayne]

Hi Adam, I think the design docs would be better if you:

1. add the mode debugging section from the official scripting wiki.

2. explain how some more complicated files work, like the hero_selection.lua, like the tutorial you wrote in your blog.

   Thanks.

[adamqqqplay]

@ArcherWayne Please check #87 , and I will add more docs in the future.