Translate documentation to Chinese

From: https://github.com/kien/ctrlp.vim/pull/506

> Hello,I translate the ctrlp doc into Chinese to help Chinese people to use this amazing plugin.
> But I am not sure whether it is a proper way to add the ctrl_cn.txt under directory /doc,if you want to use a better way,please tell me how to add the Chinese version doc.
>
> sorry，I find when the ctrlp.txt and ctrlp_cn.txt both exists,the helptags command will generate empty tags file,as most people use vundle or pathogen to manage plugins,it will be a problem,could you tell me how to solve this problem?

Comment from @kien:

> This is great. Thank you!
>
> As for the filename, I'm not sure what the language code for Chinese is. If it's "cn" then the filename should be ctrlp.cnx.

Follow-up from @codepiano:

> Yeah，you are right,it is because the file extension,I change the filename to ctrlp.cnx,and this time the :helptags command generate file 'tags' for ctrlp.txt and generate 'tags-cn' for ctrlp.cnx.In my language environment,use ':help ctrlp' command can show the correct Chinese doc file.
> I push to this branch again.