Add the instance-id design in the list of approved designs

Change-Id: I39bf46af8ced22a6e317c2a103e1139401f75c5c
diff --git a/pages/design-docs/index.md b/pages/design-docs/index.md
index b9bfa2b..2b1f1e7 100644
--- a/pages/design-docs/index.md
+++ b/pages/design-docs/index.md
@@ -22,6 +22,7 @@
 - [Scaling Multi-Master Replication](design-docs/scaling-multi-master-replication.html) by Martin Fick, approved 2019-11-12
 - [Attention Set](design-docs/attention-set.html) by Ben Rohlfs, approved 2020-01-03
 - [Change Log: Threaded Feedback](design-docs/change-log-threaded-feedback.html) by Jacek Centkowski, approved 2020-03-25
+- [Instance Id](design-docs/instance-id/index.html) by Fabio Ponciroli, approved 2020-05-05
 
 ## Reviews of Design Documents