First pass at a guide to compiler plugins

@steveklabnik, are you interested in looking this over?
Eh, in both cases the base input is a sequence of token trees.  A syntax extension plugin can parse to AST types using `syntax::parse`, but a MBE macro can do something similar with a metavariable like `$x:expr`.  The result of a MBE macro is a token tree, while the result of a syntax extension is a thing that maybe knows how to make an expression, maybe knows how to make a pattern, and so forth.

So it's all a bit murky and I don't think there's a clear distinction worth highlighting.