[README] Fix titles.
[README] Add table of contents.

I used following service to create table of contents:
DocToc http://doctoc.herokuapp.com/
[client/README] Use "##" to H2 indent instead of "----"
[client/README] Add table of contents.

I used following service to create table of contents:
DocToc http://doctoc.herokuapp.com/
[server/README] Use "##" to H2 indent instead of "----".
[server/README] Add table of contents.

I used following service to create table of contents:
DocToc http://doctoc.herokuapp.com/
[README] Fix indent.
[README] Remove first level1 caption.
[server/README] Remove first level1 caption.
[client/README] Remove first level1 caption.
[README] Fix indent.
Merge pull request #331 from project-hatohol/add-doctoc-to-README

Add table of contents to README