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!
Should use the symbol version of the splat here. This was changed recently.

```ruby
 add ["orders", :*], OrderResource
```