doc: add M88/M89 release notes

BUG=None

Change-Id: Ia5bb63a0825a4dd5aaeca305fb7796f36dd06e92
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208944
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#33359}

docs/release-notes.md

diff --git a/docs/release-notes.md b/docs/release-notes.md
index 5bb501b..5f77b9e 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -8,6 +8,8 @@
 [chromium dashboard](https://chromiumdash.appspot.com/schedule)
 
 ## List of releases
+  * [M89 Release Notes](https://groups.google.com/g/discuss-webrtc/c/Zrsn2hi8FV0/m/KIbn0EZPBQAJ)
+  * [M88 Release Notes](https://groups.google.com/g/discuss-webrtc/c/A0FjOcTW2c0/m/UAv-veyPCAAJ)
   * [M87 Release Notes](https://groups.google.com/g/discuss-webrtc/c/6VmKkCjRK0k/m/YyOTQyQ5AAAJ)
   * [M86 Release Notes](https://groups.google.com/g/discuss-webrtc/c/pKCOpi9Llyc/m/QhZjyE02BgAJ)
   * [M85 Release Notes](https://groups.google.com/d/msg/discuss-webrtc/Qq3nsR2w2HU/7WGLPscPBwAJ)