gitignore.txt: suggestions how to get literal # or ! at the beginning

gitignore.txt: suggestions how to get literal # or ! at the beginning

We support backslash escape, but we hide the details behind the phrase
"a shell glob suitable for consumption by fnmatch(3)". So it may not
be obvious how one can get literal # or ! at the beginning of pattern.
Add a few lines on how to work around the magic characters.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>