Themekit: [Doc] using `env` example

[NathanPJF]

Can an example on how to use --env be added to the documentation, please?

Just spent a good while trying to figure out why theme replace [file] --env=staging wasn't working. It would be helpful to be shown an example of using different environments.

[NathanPJF]

For adventures who make it this far, the answer: theme replace --env=staging [file]

[adamwooding]

:+1:
I just spent an hour trying to figure out how to upload to different environments & I just found this thread. Thank you SO much @NathanPJF for posting the correct way to do it. I agree that this should be added to the documentation :)

[Jorybraun]

Thank you !!!!

[eddyparkinson]

Would be great to have docs for 'env' here as well:

https://shopify.github.io/themekit/