util/e6400-flash-unlock: restore README

when nicholas added this, he removed the README because it's
going on libreboot.org instead. however, i merged a WIP version
of his page for now because i want to get the e6400 going in
libreboot sooner. so, temp-readding this README. will just
link to this on codeberg or something, from the lb docs

NOTE: I didn't write this README, hence author field set
in the commit. Nicholas wrote it, but I (Leah Rowe) am just
adding it. so, git author set to nicholas, not me