Add 2 sprints to calendar

wiki/src/contribute/calendar.mdwn

@@ -2,10 +2,6 @@
 
 All times are referenced to Berlin and Paris time.
 
-# 2019Q4
-
-* 2019-12-27, 14:00-16:00: Release Managers meeting
-
 # 2020Q1
 
 * 2020-01-07: **Release 4.2** (Firefox 68.4, bugfix release, with one exception — intrigeri is the RM)
@@ -13,8 +9,12 @@ All times are referenced to Berlin and Paris time.
     in Tails Upgrader
   - Automatic upgrade from 4.0 and 4.1, using the old upgrade system
 
+* 2020-01-08 to 2020-01-10: Illustration sprint
+
 * 2019-01-12, 17:00: Fundraising meeting
 
+* 2020-02-05 to 2020-02-12: Accounting & management sprint
+
 * 2020-02-11: **Release 4.3** (Firefox 68.5, bugfix release — anonym is the RM, intrigeri in the backup RM and helps with upgrades)
 
   Automatic upgrade from 4.2, using the new upgrade system (but still