Documentation isn't entirely clear

[sagebind]

Got some good feedback about the documentation yesterday, such as:

• Include a summary of #[unstable] in the crate-level docs
• Include more of the documentation in the readme as well
• Explain how this interacts with crate features
• Show a code example of what the macro expands to
• Create a "mock" demo module that shows what generated docs look like
  • I wonder, can a proc macro use a previous version of itself to do this?