gitattributes.txt: document how to normalize the line endings
The instructions how to normalize the line endings should have been updated
as part of commit
6523728499e 'convert: unify the "auto" handling of CRLF',
(but that part never made it into the commit).
Update the documentation in Documentation/gitattributes.txt and add
a test case in t0025.
Reported by Kristian Adrup
https://github.com/git-for-windows/git/issues/954
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>