docs: add back 'Documentation/Changes' file (as symlink)

Jaegeuk Kim reports that the debian kernel package build gets confused
by the lack of Documentation/Changes file.  We also refer to that path
name in ver_linux and various how-to files and Kconfig files.

The file got renamed away in commit 186128f7 ("docs-rst: add
documents to development-process"), and as Jaegeuk Kim points out, the
commit message for that change says "use symlinks instead of renames",
but then the commit itself actually does renames after all.

Maybe we should do the other files too, but for now this just adds the
minimal symlink back to the historical name, so that people looking for
Documentation/Changes will actually find what they are looking for, and
the debian scripts continue to work.

Reported-by: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>