Add information about all supported actions in README

[colin-nolan]

There appears to be 16 different actions, yet only 6 are listed (with little details) in the README:

['get_aliases', 'get_cname', 'get_host', 'get_network', 'get_next_available_ip', 'get_fixedaddress', 'reserve_next_available_ip', 'add_alias', 'add_cname', 'add_host', 'delete_alias', 'delete_fixedaddress', 'delete_host', 'delete_cname', 'set_name', 'set_extattr']

It would be helpful if the information in the README about the supported actions was updated.