doc: README: add example of gc query
doc: README: add example of gc query

Change-Id: I130d24d1e13549562582500aad13a4bb8646a855
Signed-off-by: Alon Bar-Lev <2e3ceb4d37adc6ae3ccc00d3646e0e0a036ace03@redhat.com>

oVirt/ovirt-engine-extension-aaa-ldap,alonbl/ovirt-engine-extension-aaa-ldap,oVirt/ovirt-engine-extension-aaa-ldap,alonbl/ovirt-engine-extension-aaa-ldap,alonbl/ovirt-engine-extension-aaa-ldap,oVirt/ovirt-engine-extension-aaa-ldap