Prepend git cloning and markdown library installation instructions.

[chivalry]

Thanks for writing this, it made it possible for me to write my vimdoc in markdown and translate it very easily. However, I'm no Python expert and the instructions caused me a few problems which I think my corrections would have eliminated. Perhaps these are obvious to more experienced Python users or perhaps just more experienced programmers. Let me know what you think.

[xolox]

Hi Charles! Improving the documentation is always useful so I merged your suggested changes. I did change the git clone URL back to my copy of the repository, given that your copy is a fork that's likely to get out of date at some point. Thanks for the contribution!