Add instructions to remove thrown error after complete with the migration.

[cromieent]

I wasn't sure what to do after running the script. The error kept popping up and my browser reported a server error. I know this is likely a silly mistake, but I'm new to Styled Components and didn't want too break anything with this script.

Therefore, I propose an additional sentence to add to the createGlobalStyles warning. Something along the lines of " Once you have hooked it up, feel free to remove this error from your code.';"

I tried adding a branch but it's locked down.

Anyhow, thanks for doing the work on this codemod. It made life easier!