Warn users that this product is unmaintained

[echlebek]

Users should be aware that this product is effectively unmaintained now. It is not receiving regular maintenance or security updates. Please update the readme to reflect this.

[sensu-discourse]

This issue has been mentioned on Sensu Community. There might be relevant details there:

https://discourse.sensu.io/t/sensu-go-is-unmaintained/3411/1