Create webpack.md

[greggb]

What: Adding documentation for bundling extracted css with webpack. May also be handy for other webpack topics in the future.

Why: tkh44 shared this snippet on Slack and it seems useful

How: Wrote a markdown file.

Checklist:

• Documentation
• Tests N/A
• Code complete N/A

Could use a once over for correctness. I left the modules: true option as that was in the example, but may need clarification on that.

[tkh44]

Thanks Gregg 🍻