Create topic for installing from USB key

We have a wiki page teaching you how to write an image to a USB key and boot from it:

https://wiki.centos.org/HowTos(2f)InstallFromUSBkey.html

This is linked from the download page:

https://centos.org/download/

Make a topic in quick-docs with this information, but update it as needed. Link to it from the download page.