docs: add missing info about Bazaar (bzr) download method

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>