doc: add info for 3.5 and 3.6 theme format versions

Author: Alberts Muktupāvels

doc/theme-format.txt

@@ -22,6 +22,18 @@ This document has separate sections for each format version. You may
 want to read the document in reverse order, since the base features
 are discussed under version 1.
 
+New Features in Theme Format Version 3.6
+========================================
+
+Add new 'invisible_border' border in frame geometry. Used for resize
+cursor area.
+
+New Features in Theme Format Version 3.5
+========================================
+
+Add support for app-menu button in window decorations. This was only
+used by Mutter and here is only to support this Theme Format Version.
+
 New Features in Theme Format Version 3.4
 ========================================