First pass at a guide to compiler plugins

@steveklabnik, are you interested in looking this over?
Could you add a code example here, the text is subtle:

```rust
// macro!() not visible to any items here

mod foo {
    #![macro_escape]

    // macro!() not visible

    macro_rules! macro { () => { println!("hello") } }

    // macro!() visible
}

// macro!() visible (including in any nested modules)
```