rails/rails — pull request #15315 by JuanitoFatas (2014-05-25 15:38:34+00:00)
Title: [ci skip] Update GFM link in guides guidelines.

Also remove a duplicated sentence:

```
, and [additional documentation](http://github.github.com/github-flavored-markdown/) on the differences from traditional Markdown.
```

This already been addressed in [GitHub Flavored Markdown](https://help.github.com/articles/github-flavored-markdown).