Add links to release schedules

wiki/src/contribute/working_together/roles/debian_maintainer.mdwn

@@ -42,3 +42,11 @@ These packages are not part of this mission:
 * [[!debpts torbrowser-launcher]]: we only use its AppArmor profiles,
   that we could easily take from upstream if the Debian package was
   not maintained.
+
+Calendar
+========
+
+* [Debian release schedule](https://www.debian.org/releases/)
+* [Ubuntu release schedule](https://wiki.ubuntu.com/ReleaseSchedule)
+  * Upcoming: BionicBeaver, 18.04, April 26th 2018
+