Documentation/notes: describe content of notes blobs

stripspace/text-based formatting kicks in when specifying the notes
content with -m or -F, or when an editor is used to edit the notes.
To binary-safely create notes from files, the following construct is
required:

git notes add -C $(git hash-object -w <file>) <object>

Explain this trick (thanks, Johan!) in the manual.  Add an ordinary
example, too, to keep this esoteric one company.

Cc: Johan Herland <johan@herland.net>
Cc: Thomas Rast <trast@student.ethz.ch>
Cc: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>