Update Readme for TypeScript support

[ezintz]

This is updating the Readme how to get TypeScript support and resolves the following issues:

• https://github.com/jaredhanson/passport-google-oauth2/issues/48
• https://github.com/jaredhanson/passport-google-oauth2/issues/23

Checklist

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have added test cases which verify the correct operation of this feature or patch.
• [x] I have added documentation pertaining to this feature or patch.
• [x] The automated test suite ($ make test) executes successfully.
• [x] The automated code linting ($ make lint) executes successfully.

Please note all elements on the checklist happend on a different repository where the type definitions are provided.

You can find the pull request here: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/35640

Kind regards

[coveralls]

Coverage remained the same at 100.0% when pulling 1ac6f22576b8c995d688fde6dc8fe23304b0dc90 on ezintz:master into aad7646d4d4413f50e2487a3e4b6df9672dbbd4f on jaredhanson:master.

[jaredhanson]

Merged, thanks!