Helpful hint: change the toggle from "Source" to "Rendered" above, and it'll show you the final output from Markdown. You might have little tweaks you want to make.

There's also a nice Sublime Text plugin called "Markdown Preview". Install it, then press Shift+Cmd+P, type "mdp", and you'll see a list of useful commands like "Markdown Preview: GitHub Flavored Markdown: Preview in Browser".