docs: adds usage example

[ethan-sargent]

• includes a simple example under Usage.
• links to the dotenv crate for details on variable substitution.

Fixes: #92

[mantmorre]

Might I suggest renaming this PR to something more verbose. Such as "Include usage examples in README.md"?

[ethan-sargent]

@aidenfarley updated to improve summary clarity, I don't think its necessary to specify which file I've changed when its the only file in the diff though.

[ethan-sargent]

@allan2 keen to get this merged and close out my sqlx PR, any blockers?

[mantmorre]

Agreed. I just included it as you had included it in the original name.

[ethan-sargent]

@aidenfarley to be honest, I just added docs to the start of the default commit message from Github, and didn't give it much consideration beyond that 😅 good call though!

[ethan-sargent]

Hey @allan2 just bumping this one. Any blockers?

[allan2]

I would prefer not to link to the unmaintained repo in this repo's README.

I have just added an example env file and linked to the variable substitution rules in this repo's README. Maybe you could reopen your PR at sqlx.