Add 2.0 betas release schedule.

wiki/src/contribute/calendar.mdwn

 [[!meta title="Calendar"]]
 
+* between 2015-11-30 and 2015-12-04: release Tails 2.0~beta1
+
+* 2015-12-11: feature freeze of Tails 2.0
+
 * 2015-12-15: Release 1.8 (anonym is the RM)
 
+* between 2015-12-15 and 2015-12-26: release Tails 2.0~beta2
+
 * 2016-01-11:
   - Build and upload Tails 2.0~rc1.
   - Start testing Tails 2.0~rc1 during late CET if building the image