doc: develop: Add a note about importing code from other projects

We talk about importing code from other projects in two places. The
first place is in the coding style section, where we explain when to or
not to deviate in terms of white space, etc. In the process
documentation we now add a note about saying where the code was imported
from and to ensure that you do not copy Signed-off-by or other tags.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>