How to add Modal in Bootstrap Sub Theme
First refer to the bootstrap documentation on modals http://getbootstrap.com/javascript/ to construct a working modal with the content you want. Use bootply or whatever to get it working the way you want. 

You then have two options that come to mind: You could just create a standard block using the Drupal UI, place the markup in the body of the block, and configure the block to "display on every page except" ... &lt;front&gt;. Then assign the block to a region. 

Alternatively, you can override tpl files in your subtheme and place the modal markup in those. You do this by placing a copy of the Bootstrap theme's page.tpl.php in your subtheme. First create a copy of it called page--front.tpl.php then add your modal markup to page.tpl.php.