add weewx-photovoltaics to README

Author: vchrisb

README.md

@@ -164,6 +164,7 @@ One limitation of the package concerns the implemented RSCP methods. This projec
 
 * [e3dc-rest](https://github.com/vchrisb/e3dc-rest): a simple REST API to access an E3/DC system
 * [e3dc-to-mqtt](https://github.com/mdhom/e3dc-to-mqtt): publish E3/DC data via MQTT
+* [weewx-photovoltaics](https://github.com/roe-dl/weewx-photovoltaics): Extension to WeeWX for processing data of the photovoltaics system E3/DC
 
 ## Contribution