http://getbootstrap.com/javascript/#modals tells you $('#myModal').modal('show')
opens a window manually. So with your modal got id="myModal", try this:
//from: https://stackoverflow.com/questions/3788125/jquery-querystring
function querystring(key) {
var re=new RegExp('(?:\\?|&)'+key+'=(.*?)(?=&|$)','gi');
var r=[], m;
while ((m=re.exec(document.location.search)) != null) r.push(m[1]);
return r;
}
var querystring = querystring('modal');
if (querystring.length && querystring[0]==='true' ) {$('#myModal').modal('show');}
This will open the modal when ?modal=true is has been set.
Reference: jQuery querystring