Update Readme
Update Readme

Added information on the OAuth Authorization Code Grant and JWT Grant examples.
The new examples also demonstrate how to use the SDK with promises.

Added information on the getUserInfo method.

Removed obsolete v2.x.x information. See prior versions of this document if they are needed.

Removed reference to the LoanCo application since it was written for an earlier version of the SDK.

Changed obsolete links from docs.docusign.com to developer.docusign.com
docusign/docusign-node-client