Added and updated documentation

I have been encouraging the use of Webmachine in the organisation that I work for as I think it is great. One of the common reasons that people give me for not using it, however, is that the learning curve is a bit steeper than for something like Grape, and that the documentation is hard to find. To this end, I've done a bit of a brain dump of docs, and reorganised them to make the README a bit more concise, but to link off to more detailed information. I've tried to explain how Webmachine works as well as give a basic example of what is required to do each HTTP method.

Feel free to edit as you see fit!
typo 2: "existance"