Test/documentation: bracketed property access is not rewritten

[domenic]

We currently do not rewrite things like

Math['max']();
Math['m' + 'ax']();

This is fine. (And perhaps even desired; if you specifically want to escape the rewriter, it provides an escape hatch.)

We should add tests showing that this does not happen, and then document this "limitation" in the readme, including referencing an ESLint rule you can use to avoid such property accesses in your project if you want to be sure the rewriter catches them all.