GitHub linker allows you to select a piece of text and create a permantent link to GitHub.com for that piece of text. Just select the text, right click, and you'll see two new options.
Copy a permanent link to the clipboard
Copy a permanent link and code as markdown
The second option not only copies the link but also copies the piece of selected text properly wrapped in a markdown code block with the programming language correctly specified. For example:
Which will be rendered as:
The file must be tracked in a git repo and the current branch must be pushed to GitHub.com.
If you've made changes to the file that are not pushed to GitHub.com, the link could be not accurate.