New option: g:startify_session_detection

doc/startify.txt

@@ -135,6 +135,14 @@ Example: let g:startify_bookmarks = [ '~/.vimrc' ]
 The number of files to list.
 
 
+============-
+
+    let g:startify_session_detection = 1
+
+When the file Session.vim is found in the current directory, it will be shown at
+the top of all lists as entry [0].
+
+
 ============-
 
     let g:startify_change_to_dir = 1