make the simple option for contributing clearer

Add more detail for beginners of how to make change directly on GitHub without needing to make a full copy of the site. 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.
Correct description of making changes directly on GitHub
Merge pull request #36 from brendam/patch-1

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