Improved the README
Improved the README

I've added a list of implemented features and I've expanded the section of the
`List images` command.

SUSE/zypper-docker,mssola/zypper-docker,jloehel/zypper-docker,mssola/zypper-docker,jloehel/zypper-docker,SUSE/zypper-docker,SUSE/zypper-docker,mssola/zypper-docker,mssola/zypper-docker,jloehel/zypper-docker