doc: README.md should have a description list for every feature.

[yuandong1222]

Such as NERDTree & Comment/UnComment.

I don't every feature, for example, I don't know what vim-surround or vim-repeat are used for.

[ketor]

Doc every feature will be a very big project. I will do this continuously.

vim-repeat is a supplement to default vim repeat by key '.', In most cases, you can ignore this feature. vim-surround is for fast operate (), """, '' {}.

A best practice for vim-surround is how to add double "" for hello world,

1. Input: Hello World 2.Enter visual mode and select Hello World
2. Enter shift+s, enter ", you will see "Hello World" 4.In normal mode, you can input cs"' to change "Hello World" to 'Hello World'.

That's all.