Files that end with the file extension .md are written in Markdown. This is a simplified markup language (HTML is also a markup language).
On GitHub, a Markdown file inside a repo can be opened and read like the file you are reading now (this is, in fact, a Markdown file), whereas the .html and .css files, etc., open in code view.
When a Markdown file on GitHub is named README.md (like this file), it will automatically open when you enter the folder it’s inside. This provides a handy way to tell people about the code in that folder.
In this case, there is no code in the folder! There's only the Markdown file. This file.
To learn how to write Markdown for GitHub, read this.
You do not need to write any Markdown files for your assignment. I just thought you might be curious about them.
If you try to read a .md file from your hard drive in your web browser, it will not work the way it does on GitHub. Basically, the file will be plain text.
If you want to read Markdown files offline, you can do it with a Chrome extension called Markdown Preview.
Be sure to check “Allow access to file URLs” in the chrome://extensions listing for this extension (go to chrome://extensions in your browser). Then you can open local .md files and read them in a well-formatted display in Chrome.
You can write and edit .md files in any text editor.
The Atom text editor has a Markdown Preview option — add it from the Packages menu, in Settings/Preferences.
Above: Atom Markdown Preview / GitHub style toggle
There’s a nice online viewer/editor for Markdown called Dillinger.