Added CHANGELOG.md generator script

[TonPC64]

Fixes

Closes #71

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 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 in line documentation to the code I modified

Short description of what this PR does:

This PR adds a generator script to update the CHANGELOG.md
See sendgrid/smtpapi-nodejs#91 for a usage and format.

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[TonPC64]

@thinkingserious why I can't recive email for get swag sendgrid hacktoberfest

[childish-sambino]

Closing as we're now using internal tooling to automate this.