Added the Table of Contents (#15218)

Author: wonntann

assets/images/help/repository/headings_toc.png

@@ -24,6 +24,11 @@ To create a heading, add one to six <kbd>#</kbd> symbols before your heading tex
 
 ![Rendered H1, H2, and H6 headings](/assets/images/help/writing/headings-rendered.png)
 
+When you use two or more headings, GitHub automatically generates a table of contents which you can access by clicking {% octicon "list-unordered" aria-label="The unordered list icon" %} within the file header. Each heading title is listed in the table of contents and you can click a title to navigate to the selected section. 
+
+![Screenshot highlighting the table of contents icon](/assets/images/help/repository/headings_toc.png)
+
+
 ## Styling text
 
 You can indicate emphasis with bold, italic, or strikethrough text in comment fields and `.md` files.