docs: Improve the mail package's README to handle the async send()

[Berkmann18]

I included the missing bits to handle the asynchronous send() call
including both ES6 and ES8 approaches.

Fixes

Closes #870

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the development 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

Short description of what this PR does:

• Adds the error handling for the send usage in the example in both ES6 (ES2015) and ES8 (ES2017).

[thinkingserious]

Hello @Berkmann18,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX

[Berkmann18]

@thinkingserious You're welcome :).