Add HOWTO section on include files

Signed-off-by: Jens Axboe <axboe@fb.com>
bloom: always use a larger minimum size for bloom filter

Increases accuracy at the lower end.

Signed-off-by: Jens Axboe <axboe@fb.com>
Clarify that include files may not contain job sections

Signed-off-by: Jens Axboe <axboe@fb.com>