Document /Set/CurrentMaxSumMains topic usage

Added information about the /Set/CurrentMaxSumMains topic for setting the Capacity.

Author: mstegen

docs/configuration.md

@@ -277,6 +277,8 @@ mosquitto_pub  -h ip-of-mosquitto-server -u username -P password -t 'SmartEVSE-x
 
 You can find test scripts in the [test directory](https://github.com/SmartEVSE/SmartEVSE-3/tree/master/SmartEVSE-3/test) that feed EV and MainsMeter data to your MQTT server.
 
+The `/Set/CurrentMaxSumMains` topic can be used to set the [CAPACITY](#capacity) setting.
+
 # Multiple SmartEVSE controllers on one mains supply (Power Share)
 Up to eight SmartEVSE modules can share one mains supply.
   - Hardware connections