inventory: add github-bot reminder when changing Jenkins worker IPs

The github-bot has a whitelist of IPs that's allowed to push Jenkins
job status as inline PR status on github.com. Whenever we change our
Jenkins worker IPs, we should remember to update the bot's whitelist
as well.

ansible/inventory.yml

@@ -231,6 +231,8 @@ hosts:
         centos7-arm64-2: {ip: 147.75.74.254}
         ubuntu1604-arm64-1: {ip: 147.75.77.130}
         ubuntu1604-arm64-2: {ip: 147.75.74.174}
+# when adding, removing or changing the IPs below,
+# remember to update Jenkins worker IP whitelist in github-bot
         ubuntu1604-x64-1: {ip: 147.75.69.113, alias: jenkins-workspace-1}
         ubuntu1604-x64-2: {ip: 147.75.73.189, alias: jenkins-workspace-2}