Global message for Sharepoint 2013 (foundation)

Hello,

we are about to replace old sharepoint with sharepoint online.

We want to keep the old SP version on for archiving for a limited time, but at the same time show a global message that "This version of Sharepoint is not supported anymore" or something like that.

Is there a easy way to put a global message on all pages that is visible for all SP 2013 visitors?

Thank you!