Add duofern.set_update_interval to docs.

Author: gluap

readme.md

@@ -55,4 +55,7 @@ Ask duofern devices to re-send their state in case. Can be used in setups where
 Dump the current last received state for all duofern modules as a warning level message to the log. This reflects the current state of all RF messages received from devices - What's not here wasn't received by the stick or came in garbled.
 
 ``duofern.sync_devices``
-Write the duofern config file with the known devices. normally not required from the user.
+Write the duofern config file with the known devices. normally not required from the user.
+
+``duofern.set_update_interval``
+Set the automatically broadcasting a "please send an update" interval. Be aware that this is not persisted. Use an automation to trigger this at Homeassistant start if you always want it to be set to your custom value.