You’ll notice there’s a new Markdown text-formatting toolbar on all the comment fields throughout GitHub. While you've always been able to use Markdown to format your text with links, headers, italics, and lists, the new toolbar allows you to do so without learning Markdown syntax.
The toolbar includes a limited set of tools that gets out of the way of experts, but helps those new to GitHub to write just as clearly, and expressively as anyone else. Neat.
In addition to standard Markdown formatting, the toolbar also includes GitHub-specific features. @mentions bring additional users or teams into the conversation, issue and pull request links allow you to cross-reference related discussions, and task lists track outstanding tasks. They’re now available to you with a single click.