Add a link

Author: jeroen

README.md

@@ -14,8 +14,9 @@ and strikethrough text.
 
 ## Documentation
 
- - [commonmark R package](https://docs.ropensci.org/commonmark/)
+ - [The `commonmark` R package](https://docs.ropensci.org/commonmark/)
  - [CommonMark specification for Markdown text](https://commonmark.org/help/)
+ - [Extensions for GitHub Flavored Markdown](https://github.github.com/gfm/)
 
 ## Basic Markdown