docs: Update docs for domain or email verification

[GeoFro]

If you are just testing this out in development and follow the instructions either here on github or inside the sendgrid web app it never mentions you must first verify a "from" domain or single address.

You will simply receive a 403 forbidden response on trying to send any mail, including the example email.
The error response does contain an error message though by default it won't be visible due to the way node simplifies deeply nested objects in console.log, as you can see below.

The actual error message does display if you know where to look error.response.body but at first glance and from the current docs you just think maybe your API keys aren't set up right.
One or two lines in the docs should prevent this.

ResponseError: Forbidden
    at node_modules/@sendgrid/client/src/classes/client.js:133:29
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5) {
  code: 403,
  response: {
    headers: {
      server: 'nginx',
      date: 'Fri, 12 Jun 2020 23:54:10 GMT',
      'content-type': 'application/json',
      'content-length': '281',
      connection: 'close',
      'access-control-allow-origin': 'https://sendgrid.api-docs.io',
      'access-control-allow-methods': 'POST',
      'access-control-allow-headers': 'Authorization, Content-Type, On-behalf-of, x-sg-elas-acl',
      'access-control-max-age': '600',
      'x-no-cors-reason': 'https://sendgrid.com/docs/Classroom/Basics/API/cors.html'
    },
    body: { errors: [Array] }
  }
}

I think some people will benefit from this PR including probably some people who found their way here

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[eshanholtz]

LGTM! Thanks so much for contributing!