docs: added a list of known extensions

docs/extensions-list.md

@@ -0,0 +1,24 @@
+## Official
+
+* [twitter-extension][1] - Adds support of Twitter usernames and hastags
+* [prettify-extension][2] - Adds [Google Prettify][3] hints to HTML output
+
+## Community
+
+* [showdown-icon][4] - Adds support of Glyphicon and font-awesome into Markdown
+* [showdown-xss-filter][5] - Filters XSS, using leizongmin/js-xss
+* [showdown-toc][6] - Adds Table of Contents
+* [showdown-footnotes][7] - Adds simple footnotes
+* [katex-latex][8] - Displays math using KaTeX and LaTeX or AsciiMath
+
+!!! note ""
+    If you have a Showdown extension you would like to add here, you can [raise an issue](https://github.com/showdownjs/showdown/issues).
+
+[1]: https://github.com/showdownjs/twitter-extension
+[2]: https://github.com/showdownjs/prettify-extension
+[3]: https://github.com/googlearchive/code-prettify
+[4]: https://github.com/dbtek/showdown-icon
+[5]: https://github.com/VisionistInc/showdown-xss-filter
+[6]: https://github.com/ravisorg/showdown-toc
+[7]: https://github.com/Kriegslustig/showdown-footnotes
+[8]: https://obedm503.github.io/showdown-katex

mkdocs.yml

@@ -42,4 +42,5 @@ nav:
   - Extensions:
     - Overview: extensions.md
     - Create an extension: create-extension.md
+    - List of known extensions: extensions-list.md 
   - Tutorials: tutorials/index.md