Add support warning to README.

Author: kattni

README.rst

@@ -13,6 +13,9 @@ Introduction
     :target: https://github.com/adafruit/Adafruit_CircuitPython_ESP_ATcontrol/actions/
     :alt: Build Status
 
+This library is no longer supported! The library is being left available for continued usage, however, Adafruit is no longer supporting it.
+=============
+
 Use the ESP AT command sent to communicate with the Interwebs. Its slow, but works to get data into CircuitPython
 
 Command set: https://www.espressif.com/sites/default/files/documentation/4a-esp8266_at_instruction_set_en.pdf