Add note to avoid confusion when following the book

[coelhucas]

Summary

While following the book I had this small issue due to newer versions of Node.js not working on older Webpack.

Instead of reworking the template with a newer Webpack version I decided to not only add how to solve, but also a quick overview that might interest other seasoned developers reading.

Post due diligence acknowledgments (edit): Fixes #290, #279 and #273.

As pointed by @nik0sc here, another possible fix would be https://github.com/rustwasm/create-wasm-app/pull/207, but since many people use older webpack versions & due to the amount of change needed in the other solution, I think this is enough for learning purposes.