Add table of contents to README

@yoshfuji san says, he think cause of reading the documents to hard is that these documents don't have table of contents.
So I add table of contents to README.

If these file have no problem, please merge this.
I fix indent at cf422cc revision.

> The second level captions are really needed ?

I think it is good to have.
Because, I think it is good to have some information in table of contents.