Readme: update dependencies section
Readme: update dependencies section

* koji is also available via pip now, it's covered by requirements files
* docker-py too (and will be removed soon)

Signed-off-by: Martin Basti <388356a3851efe39ff0604de6d8fb2e16e10a0b7@redhat.com>

fr34k8/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor