misc: Update RELEASE-NOTES.md for v21.2.1.1

Change-Id: I35e511a33fb4fd0327188602174ccb906efbb0eb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59271
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

RELEASE-NOTES.md

diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 156494b..5d569b4 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -1,3 +1,8 @@
+# Version 21.2.1.1
+
+**[HOTFIX]** In order to ensure v21 of gem5 remains compatible with future changes, the gem5 stdlib downloader has been updated to obtain the resources.json file from <https://resources.gem5.org/resources.json>.
+As this domain is under the gem5 project control, unlike the previous googlesource URL, we can ensure longer-term stability.
+The fix also ensures the downloader can parse plain-text JSON and base64 encoding of the resources.json file.
 
 # Version 21.2.1.0