Flesh out the README some more.

Add more detail to the "writing a post" section, and add a new
"contributing your post" section dealing with the business of pull requests.
Make "fork this repository" a link.

Actually fork the repository when this link is clicked.
Add a link to rubyinstaller.org for Windows.
Remove details of setting up GitHub pages.

This isn't necessary for a contributory repository, only the master one.
Add details of previewing posts.