.gitignore: don't ignore .gitattributes files

.gitignore: don't ignore .gitattributes files

In a private fork, it's not unreasonable to commit various binary
files associated to one's project, and thus put a suitable
.gitattributes file next to it to force git to treat that file as
binary. In my case, I have a .xslx spreadsheet used for computing
suitable RAM timing tables, which I wanted to put in
board/<vendor>/<project>. Git duly warned me that it would do LF/CRLF
mangling, so I also added a .gitattributes file next to it with

  *.xlsx binary

but upon adding that file, git told me that I'd have to use -f because
it's a dot-file that is by default ignored.

Add .gitattributes to the list of dot-files that should not be
ignored.

While in here, sort the list and update the comment, as there are also
files which git itself does not make use of in the list, cf. linux
commit f46e65da48b2 (".gitignore: exclude .get_maintainer.ignore and
.gitattributes").

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>