First pass at a guide to compiler plugins

@steveklabnik, are you interested in looking this over?
A procedural macro really just manipulates token trees to possibly *output* some AST (e.g. https://github.com/huonw/compile_msg expands to nothing meaningful); there's [no particular need](https://github.com/huonw/brainfuck_macros) for it to be interpreting the input as an actual Rust AST.