Mention Google maven repo on public keys page

The maintainers also sign artifacts that are published to the
Google Maven repository.

Change-Id: Ib888c40a48d722c45218c772e21015e54b306ff2

releases/public-keys.md

diff --git a/releases/public-keys.md b/releases/public-keys.md
index e9c1b34..1687f12 100644
--- a/releases/public-keys.md
+++ b/releases/public-keys.md
@@ -4,7 +4,7 @@
 [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 artifacts that are published on the Maven Central and Google Maven repositories
 * Release announcement emails
 * Binary package distributions