updated Readme.md with better details

This relates to:
https://github.com/GigaOM/legacy-pro/issues/2875
https://github.com/GigaOM/legacy-pro/issues/2859

This was built to support [GIgaom Alerts](https://github.com/GigaOM/go-alerts).

A consolidated list of features for this plugin does not exist as the base functionality was already part of a few other plugins:
- [GIgaom Events](https://github.com/GigaOM/gigaom-plugins/tree/master/go-events), 
- [GIgaom Event Navigation](https://github.com/GigaOM/gigaom-plugins/tree/master/go-event-navigation)
- [Gigaom Webinars](https://github.com/GigaOM/gigaom-plugins/tree/master/go-webinars])