Update usage README
Update usage README

Add `unicorn` gem description

dunnock/dockerfiles,jasonvillalon/dockerfiles,seapy/dockerfiles,seapy/dockerfiles,stormtoy/dockerfiles,amos6224/dockerfiles,amos6224/dockerfiles,dmsilaev/dockerfiles,jasonvillalon/dockerfiles,rafael/dockerfiles,rafael/dockerfiles,dunnock/dockerfiles,stormtoy/dockerfiles,dmsilaev/dockerfiles