Update keys page to mention signing tags

Change-Id: If1fa44c88e0b40089d2d673dafe134f1d48f4503
diff --git a/releases/public-keys.md b/releases/public-keys.md
index 62fc983..c268595 100644
--- a/releases/public-keys.md
+++ b/releases/public-keys.md
@@ -3,6 +3,7 @@
 The following public keys are are available on the
 [Ubuntu Public Key Server](https://keyserver.ubuntu.com/), and are used to sign:
 
+* Release tags on the gerrit and core plugin repositories
 * Release artifacts that are deployed to Maven Central
 * Release announcement emails
 * Binary package distributions