The git add --chmod option was introduced in 4e55ed32 ("add: add
--chmod=+x / --chmod=-x options", 2016-05-31), but was never
documented.  Document the feature.
Signed-off-by: NThomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>