Update README.md to try to make it easier for people new to github

Add more detail for beginners of how to make change directly on GitHub without needing to make a full copy of the site. Wasn't as simple as I though, so maybe they are still too long. Be good if it was possible to use the images from the github help pages to make things clearer.

 Might want to consider moving the section on making a development server to a separate document. Better instructions for prose.io would be good too.