add version docu for ICS

doc/source/ics.md

@@ -47,6 +47,7 @@ waste_collection_schedule:
         params: PARAMS
         year_field: YEAR_FIELD
         split_at: SPLIT_AT
+        version: 2
 ```
 
 ### Configuration Variables
@@ -100,6 +101,14 @@ Field in params dictionary to be replaced with current year (4 digits including
 
 Delimiter to split event summary into individual collection types. If your service puts multiple collections types which occur at the same day into a single event, this option can be used to separate the collection types again.
 
+**version**<br>
+*(integer) (optional, default: 2)*
+
+Selects the underlying ICS file parser:
+
+- version: 1 uses `recurring_ical_events`
+- version: 2 uses `icalevents`
+
 ## Examples and Notes
 
 ***