Add documentation on how to use a selector negation
Add documentation on how to use a selector negation

This will help users use the `--selector` flag to selectively exclude objects from being backed up by ark

workaround for #404 until dedicated flags are implemented

Signed-off-by: Dhananjay Sathe <521464818a6784d05dbb0bac81da62bfc40aae44@gmail.com>

heptio/ark,heptio/ark