Improve migrate.org and mention it in the README

[edgar-vincent]

I was surprised to see that the README stated that no-littering could not help the user migrate their files while, actually, there was a handy migration guide in the repository. Moreover, some steps in the guide seemed unclear to me. This PR thus:

• Mentions the migrate.org file in the README and in the comments at the top of no-litter.el.
• Improves the way the results of the Elisp Babel block containing the migration script generator are printed in the org file.
• Improves the wording and the spelling in that same file.

[tarsius]

Thanks!

I've reworded/replaced the main commit.