You could use app_offline.htm and create the page under your root site, that should do the trick.
http://www.sharepointdiary.com/2012/07/maintenance-page-for-sharepoint-2010.html
if you make your site readonly, then you will not be able to make changes to the site, the site will be locked for update/edit. Being said that, none of the users including you will not be able to make changes.
Hope this helps!
Ram - SharePoint Architect
Blog - SharePointDeveloper.in
Please vote or mark your question answered, if my reply helps you