misc: Update release notes for v21.1.0.1 Hotfix

Change-Id: I8a99b2602b1fce566006a4e07aee8c3b574a6ae1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50030
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 480e4fe..a3517c1 100644
@@ -1,3 +1,7 @@
+# Version
+**[HOTFIX]** [A "'deprecated' attribute directive ignored" warning was being thrown frequently when trying to build v21.1.0.0](https://gem5.atlassian.net/browse/GEM5-1063). While this issue did not break the build, it made reading the build output difficult and caused confused. As such a patch has been applied to fix this issue.
 # Version
 Since v21.0 we have received 780 commits with 48 unique contributors, closing 64 issues on our [Jira Issue Tracker](https://gem5.atlassian.net/).