Drop rpm-py-installer from requires
    
    rpm-py-installer is removed from requires.
    
    Add a new section to documentation for how to install rpkg, which
    mentions to install rpm-py-installer with rpkg together.
    
    For running tests, rpm-py-installer is added to deps option.
    
    Fixes #357
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>