README updates
README updates

- Note difference in ports between RabbitMQ 2.x and 3.x
- Add description in README of some recently added checks

nagios-plugins-rabbitmq/nagios-plugins-rabbitmq,nagios-plugins-rabbitmq/nagios-plugins-rabbitmq,jamesc/nagios-plugins-rabbitmq,crsuarez/nagios-plugins-rabbitmq,themiszamani/nagios-plugins-rabbitmq