Getting Started instructions added to README.md

This is how I would address #11 (closed). I figured it was easier to submit a PR that could be discussed than trying to discuss what I was thinking there.

This adds instructions for both forking this repository and starting from scratch, making it easier for users to take the path they are more comfortable with.

If they take the from scratch route, they know they will be getting bog standard Jekyll. If they fork, then they know they are getting something that may contain others' work and (possibly) customizations. For this reason, it think it is acceptable that it does not use a gem-based theme even though that is what you get when you start from scratch. The result is that non-gem based themes are used (as was indicated to be the preference by @marcia and @axil), there no need for forks, and this repository can continue to be updated to build against the latest version of Jekyll.