Fix gen-typescript-declarations docs

[nikolay-borzov]

It says

Add a gen-tsd script to your package.json:

{
  ...
  "scripts": {
    "gen-tsd": "gen-tsd"
  }
}

But actual command file name is gen-typescript-declarations

[aomarks]

Thanks for noticing, fixed the README, and added some other recent updates too (e.g. Polymer 3).