doc updates

Author: dreed47

README.md

@@ -9,6 +9,8 @@
 
 <img src="https://raw.githubusercontent.com/home-assistant/brands/master/custom_integrations/redfin/logo.png" width="40%">
 
+### NB!: This component requires HA Core version 2021.6.0.dev0 or greater!
+
 This is a _Custom Integration_ for [Home Assistant](https://www.home-assistant.io/). It uses the unofficial [Redfin](https://www.redfin.com) API to get property value estimates.
 
 There currently is support for the Sensor device type within Home Assistant.