[patch] Explain how to exit and clean up a guix environment

[Lee Fallat]

Hey,

It'd be useful to explicitly state how to do this. While my guess of
exiting the shell (or ctrl-d) was correct, not everyone will know!

Also there are people like myself who like to reduce storage usage if
possible on machines which don't have much, like on netbooks or ultra-
books. I've mentioned how to remove packages after exiting from the
environment.

I hope I'm submitting this patch correctly, if not let me know!

-- Lee

guix_environment_exit_and_clean.patch
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