contributing: document the release cycle freezes

These should be the conventions we have been using since 1.0, written
down more accurately.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>
1 job for master in 2 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #9592
build-native

00:02:03