Update README.md for ENV vars with dev server
Update README.md for ENV vars with dev server

Replaces information about dev server CLI flags with new environment variables usage when running webpack-dev-server
usertesting/webpacker,rossta/webpacker,gauravtiwari/webpacker,gauravtiwari/webpacker,usertesting/webpacker,rossta/webpacker,rossta/webpacker,gauravtiwari/webpacker,rails/webpacker,usertesting/webpacker,rossta/webpacker,rails/webpacker