Update documentation/CONTRIBUTING.md

Co-authored-by: Anastasia Alexandrova <[email protected]>

documentation/CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 `pg_tde` documentation is written in Markdown language, so you can [edit it online via GitHub](#edit-documentation-online-vi-github). If you wish to have more control over the doc process, jump to how to [edit documentation locally](#edit-documentation-locally). 
 
-Before you start, learn what [git], [MkDocs] and [Docker] are and what [Markdown] is and how to write it. For your convenience, there's also a cheat sheet to help you with the syntax. 
+Before you start, learn what [git], [MkDocs] and [Docker] are and what [Markdown] is and how to write it. For your convenience, there's also a [cheat sheet](https://www.markdownguide.org/cheat-sheet/) to help you with the syntax. 
 
 The doc files are in the `documentation` directory.