docs: add another 20.1.x release to allow for more overlap with 20.2

The aim is to have the last release of the old branch on or after the .1
release on new one, but as things stand the last 20.1 would happen
between 20.2.0 and 20.2.1, so let's add one more.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6615>
diff --git a/docs/release-calendar.rst b/docs/release-calendar.rst
index 2fe677c..69fd033 100644
--- a/docs/release-calendar.rst
+++ b/docs/release-calendar.rst
@@ -30,7 +30,9 @@
 +--------+---------------+------------+-----------------+-----------------------------------------+
 | Branch | Expected date | Release    | Release manager | Notes                                   |
 +========+===============+============+=================+=========================================+
-| 20.1   | 2020-09-16    | 20.1.8     | Eric Engestrom  | Last planned release of the 20.1 series |
+| 20.1   | 2020-09-16    | 20.1.8     | Eric Engestrom  |                                         |
+|        +---------------+------------+-----------------+-----------------------------------------+
+|        | 2020-09-30    | 20.1.9     | Eric Engestrom  | Last planned release of the 20.1 series |
 +--------+---------------+------------+-----------------+-----------------------------------------+
 | 20.2   | 2020-09-02    | 20.2.0-rc3 | Dylan Baker     |                                         |
 |        +---------------+------------+-----------------+-----------------------------------------+