Changed README to Markdown

First of all this project looks awesome and I'm super excited to dig into it. 

In an effort to help (and to make it a bit easier to read) I changed your README over to Markdown format. This renders in a nicer way on GitHub, and really is the standard. I didn't change any content (although there may be be a few places where that would make sense such as pointing issue tracking to https://github.com/google/rekall/issues). Let me know if I can make any further changes or improvements. :sparkles: