Add integrate to nnn

[shuizhongyueming]

I have mad a pull request to add support of z.lua in nnn's plugin autojump: jarun/nnn#1414.

I think is good to add the usage in z.lua's wiki, but I have no edit permisson, below is the content I want to add in FAQ blow the topic of integrate to ranger:

## How to integrate z.lua to [nnn](https://github.com/jarun/nnn) ?

The z.lua was integrated into the nnn's plugin autojump.

All you need to do is:

\`\`\`bash
export NNN_ZLUA="path to your z.lua script"
\`\`\`

so that, the plugin can know where to get the z.lua script to excute.

After that, you can enable plugin autojump in `$NNN_PLUG`.

And if `fzf` is installed, the plugin will use it for path selection which is copy from the `zii` function below this document.