docs/release-calendar: Add calendar for 20.1 Release candidates

docs/release-calendar: Add calendar for 20.1 Release candidates

It's time to start getting the calendar going for 20.1 so that everyone
is clear on when the close date for new features is. Eric Engstrom has
agreed to help out with the 20.1 series, and will be the primary point,
he's also helping out with a few of the 20.0.x point releases.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4077>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4077>