Add link to Gerrit builds on CI

To make the Gerrit builds more discoverable over
the internet, it is better to add a link from the Gerrit
home page.

This allows the CI to be indexed by Google Search engine
and people to easily find pre-built version of their plugins.

Change-Id: I5a4b4dfc25df572d7c0fb17b72dc46d05b31de0b