grub2: move usage notes to package readme.txt

grub2: move usage notes to package readme.txt

As discussed in the mailing list, grub2 usage notes were growing too big
for a Config.in documentation, and so it was agreed that a readme.txt in
the package directory is a better place to put them.

This commit simply moves the documentation as-is to preserve the
original contents as they were in Config.in which can be worked on in
further commits.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>