document dev setup to progressively migrate transitional backend driven app to SPA with Aurelia

[3cp]

To hook up backend

• customize aurelia_project/tasks/run.js using http-proxy-middleware
  • support both http/https backend
  • explain connect-history-api-fallback and http-proxy-middleware
  • backend api (to support new front-end) convention to get easy dev setup
• customize aurelia LinkHandler to force full reload to deal with un-migrated old pages.
• router config trick to deal with un-migrated pages.
• notes on progressively migrating page by page