New docs article about creating custom Server VM needs review

[Peter Boy]

I wrote a new documentation about creating (custom) Server VMs using ImageFactory. 

ImageFactory – How to Create a Virtual Machine Disk Image
https://docs.stg.fedoraproject.org/en-US/fedora-server/tutorials/imagefac...

See issue
https://pagure.io/fedora-server/issue/100 

We need a review.