Update notes on including 6to5 runtime.

Sorry about this - the original documentation I wrote was in fact wrong. The 6to5 runtime doesn't export, but rather sets `global.6to5Runtime` in a somewhat complex series of IIFEs so that it will work in both page and webworker contexts.

To get around this, you can either `require('6to5/runtime')` in your entry point, or pass it to each module using the ProvidePlugin. It's a bit janky as you have to use `exports-loader` to grab the export, and `imports-loader` to prevent it from being exported to the window, but it's the only way to prevent the leakage given how the runtime is written.