docs/manual: add a 'Known issues' chapter

Add a 'Known issues' chapter, which lists the exceptions I have
currently in the autobuilder scripts to avoid known problems from
occuring. I believe it is more useful to document them rather than
keeping them hidden in my autobuilder script.

[Peter: s/either/either use/]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>