add note to keep _info and events in sync

en/events.md

 ---
 title: Events
 ---
+<!-- Please keep ongoing and current events in sync with _index.md -->
 - Ongoing:
    - **card10 reworking/soldering/etc.**: card10 village (R8)
 - Upcoming:
    - **card10 firmware workshop**: card10 village (R8), Sun 18.08. 16:00
    - **card10 [talk](https://fahrplan.events.ccc.de/camp/2019/Fahrplan/events/10365.html)**: Curie Wed 21.08. 12:00
+
+<!-- past events, please remove any events you move here from _index.md to keep the home page clean -->
 - Previous
  - card10 Soldering Session in Berlin: xHain Wed-Thurs 14&15. August starting at 10 am
  - card10 Soldering Session in Berlin: xHain Sat-Sun 10.-11. August starting at 10 am