Update README.md

[avdata99]

Fixes

A short description of what this PR does.

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 main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket.

[palfrey]

I'd like to file an issue noting that using mypy with this gets you error: Skipping analyzing "sendgrid.helpers.mail": module is installed, but missing library stubs or py.typed marker [import-untyped] but well, issues!