When you're deploying to Github, crafting a helpful and well-formatted README file is a great starting point for people interested in your project. There are lots of Markdown preview tools available (just check for plugins or bundles in your favorite editor/IDE, or an online tool like http://markable.in/editor/), but Github does things a little differently with their Markdown flavor and it can be difficult to get it the way you want it without pushing to Github and seeing what needs to be changed.
That's where i've found a tool called grip to be really helpful. From their documentation, it is a "command-line server application written in Python that uses the Github Markdown API to render a local readme file".
To install Grip you first need a python package manager called pip. If you are on Mac OS X or Linux, you probably already have Python installed and pip should be available. If on windows, you'll need to install Python. If you're having trouble getting setup, try the installation instructions here.
Once you have pip, it's as simple as pip install grip.
Now that you have it installed, simply run grip in the same folder as your README and it'll be served up and available in your browser. The default is http://localhost:5000. Now enjoy previewing your Github Markdown before pushing to the repo!