Update Svelte.js/README.md - use npx and update tailwind css config file name

[paul42]

use npx tailwindcss init and update file name

also, should the dev command be like that? what is 'run-p' and 'autobuild' ? for reference, the most recent svelte template package.json lines from the template (after 08/18/2020) updated to the latest TS stuff:

 "build": "rollup -c",
 "dev": "rollup -c -w",