Add link to a custom dashboard card to the readme

This update provides no change in functionality but just introduces a very nice dashboard card created by another user (@seevee) for use in displaying the sensor data in your frontend.

The card is located here and can be installed into Home Assistant via HACS (search for "weather alerts card").

Just to clarify, I'm not the creator or maintainer of that card so any issues or feature requests should be directed to their Github page and not here.