docs: Add link to online demo

Author: KEINOS

README.md

@@ -1,6 +1,9 @@
 # Listing Table of Contents Extension/Plugin for [Parsedown](http://parsedown.org/)
 
 This simple PHP file extends [Parsedown (vanilla)](https://github.com/erusev/parsedown) to generate a list of table of contents, aka ToC, from a markdown text given.
+## Online Demo
+
+https://paiza.io/projects/0TghplxParLqyrP1tjAg6g
 
 ## Install