Add blit changes to migration guide.

I was following https://jimp-dev.github.io/jimp/guides/migrate-to-v1/ to upgrade my script and for the most part it was really helpful.

What was missing for me was the change in image.blit() from multiple parameters to a single option object.

Would that be something to include on that page? Are there other methods that changed in the same way?

Start with the migrate-to-v1 guide at https://jimp-dev.github.io/jimp/guides/migrate-to-v1/ and verify how image.blit() changed from multiple parameters to one options object. Check whether other methods made the same kind of change, then update the guide with the relevant migration details and confirm the page accurately reflects the v1 API.

Written by the indexing model from the issue text.