Add Example use cases with verify_sql in README

[trevorturk]

Description

Closes #91

Adds an "Example use cases" section to the README with an example verify_sql Rspec helper for use in Rails apps with complex SQL queries.

I was searching for the old code sample from this issue: #91 and thought it worth trying to write something up in the docs so it's easy (for me) to find (lol) next time.

Please feel free to discard, or perhaps consider opening the wiki or discussions if it might be better to post stuff like this somewhere else. (Apologies, it looks like you're trying to keep the README small, so I feel bad even proposing this!)

[geeksam]

Hello, and thank you! Katrina handed this gem off to @isidore and me earlier this year. Any terseness you see in the README is from years ago; I, for one, am a big fan of verbose READMEs. :)

We've both been super busy, but I wanted to let you know I'd seen this, and I hope to get to it soon...ish...

[trevorturk]

No worries at all, and don't worry if you prefer to handle this some other way (or not!) I'm just a fan of this idea and wish more people knew about it, especially when using GraphQL it's very handy. Thanks again in any case!