Sendgrid-nodejs: Link to docs from README

[kaycebasques]

Hi, Chrome DevTools technical writer here. I'm using your fine services in a personal project.

Suggestion: Put a link to the docs, front-and-center, at the top of this repo's README.

[thinkingserious]

Hi @kaycebasques,

If you don't mind, PRs are welcome :)

If not, no worries, I've added this to our backlog.

With Best Regards,

Elmer