Merge branch 'markdown-attribution' into 'master'

Attribution where it is due.

Inspired by https://news.ycombinator.com/item?id=11431969

See merge request !3546

doc/markdown/markdown.md

@@ -31,7 +31,7 @@
 
 _GitLab uses the [Redcarpet Ruby library][redcarpet] for Markdown processing._
 
-For GitLab we developed something we call "GitLab Flavored Markdown" (GFM). It extends the standard Markdown in a few significant ways to add some useful functionality.
+For GitLab uses "GitLab Flavored Markdown" (GFM). It extends the standard Markdown in a few significant ways to add some useful functionality. It was inspired by [GitHub Flavored Markdown](https://help.github.com/articles/basic-writing-and-formatting-syntax/).
 
 You can use GFM in