Add Details to Configurable Items in the Templates Generated

After providing the code in the generate-sw.js file the extension allows you to tab through and modify some of the contents that were generated (e.g. BUILD_DIR, SW). It would be nice to have some documentation describing what the extension tools suggests you modify, what they're used for/why we might modify what was generated.

This applies across the tools and not just the pwa-generate-service-worker piece.