Add details to readme file

[mohsen1]

I didn't know this repo has a CLI or I can download all images easily so I wrote my own script to download them all. It would be nice if there was more details in readme to explain its features

[stephan281094]

+1, I want to download all images, but I don't know the cli works.
I tried `node cli.js --download="/path/to/my/folder" but it returned an error 'ENAMETOOLONG'.

Some documentation would be greatly appreciated!

[mohsen1]

In case you are interested in the script it's here:
https://gist.github.com/mohsen1/2bba3165ade2c4016c10

[stephan281094]

Oh that's awesome, thanks!

[jorioux]

Here is a powershell script I made to download the backgrounds :

https://gist.github.com/jorioux/c3ed39bc71682177a80c80a790b20f80