docs: Create USE_CASES.md

[flaredragon]

Fixes #60

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:

• Creates USE_CASES.md file.

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @flaredragon,

Thanks again for the PR!

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

Team SendGrid DX

[mbernier]

This is awesome!