International friendly README

As mentioned in #14 .

I also updated the `lang` attribute in html template because it will cause annoying translating message on Chrome:
![image](https://cloud.githubusercontent.com/assets/2840571/3732039/f9b90dde-16f5-11e4-9b2c-56d4c0f20572.png)

I want you to change the description.

```
2. Start editing the file of Markdown.(`filetype` is `markdown`)
    * __NOTE__: In the case of `.md`, `filetype` becomes a `modula2`. If so, please describe in `.vimrc` this setting
    ```vim
    augroup PrevimSettings
        autocmd!
        autocmd BufNewFile,BufRead *.{md,mdwn,mkd,mkdn,mark*} set filetype=markdown
    augroup END
    ```
```