Update release-notes/1.20.1.md

Add known issue and workaround for Android 11 containers failing to start with pure software rendering.

Co-authored-by: Alexis Janon <alexis.janon@gmail.com>

release-notes/1.20.1.md

@@ -25,3 +25,17 @@ Please see [component versions](https://anbox-cloud.io/docs/ref/component-versio
 ## Upgrade instructions
 
 See [Upgrade Anbox Cloud](https://anbox-cloud.io/docs/howto/update/upgrade-anbox) or [Upgrade the Anbox Cloud Appliance](https://anbox-cloud.io/docs/howto/update/upgrade-appliance) for instructions on how to update your Anbox Cloud deployment to the 1.20.1 release.
+
+## Known issues
+
+Since 1.20.0, Android 11 containers fail to start with pure software rendering (i.e., without any GPU attached) because of a missing library. This will be fixed in 1.20.2.
+
+Affected users can solve the issue by installing the missing package in a pre-start [hook](https://anbox-cloud.io/docs/ref/hooks):
+```
+#!/bin/sh -ex
+if  [ "$CONTAINER_TYPE" = "regular" ]; then
+  exit 0
+fi
+apt update
+apt install --no-install-recommends -y libgl1-mesa-dri
+```