Guide user to install theme as submodule

[WordlessEcho]

Hexo recommends user to edit theme config by theme_config:

theme_config:
  # ...

However, it encountered some problem in removing entries of menu. Because Hexo will deep merge config instead of replace config:

theme_config:
  menu:
    Home: /

Will still be this in Seje:

{
  "menu": {
    "Home": "/",
    "Archives": "/archives",
    // ...
}

Hexo recommends the config of theme should be comment that all:

menu:
  Home: /
  # Archives: /archives
  # About: /about
  # RSS: /rss
  # Member: /member