Add docs for easy-image-cli

[wwalc]

Mention how to download it etc.

https://github.com/cksource/bigbang-docs.ckeditor.com/tree/master/projects/cs/docs/guides/easy-image

More or less what we have in readme.

[SlawekGreczyn]

https://github.com/cksource/bigbang-docs.ckeditor.com/blob/easyimagecli/projects/cs/docs/guides/easy-image/easy-image-cli.md