Markdown is an enriched text, human readable, format. You can open a .md file using any text editor. However, you will see the source text, without direct access to links and images.
Browsers are able to render Markdown, but usually require plugins (free).
In Chrome [menu], select [More tools], [Extensions].
Select the option Get more extensions to access Google Web Store
Search for Markdown Viewer and click on Add to Chrome
Finally, go back to the Extension menu of Chrome. In the Markdown Viewer entry, select [Allow access to file URLs]
You should now be able to read Markdown file from the browser.
Visual Code Studio has an built-in Markdown viewer accessible through the following icon.
Atom has also built-in Markdown viewer through the following menu (or Ctrl+Shift+M shortcut)