I've just updated DESIGN and PACKAGING documents for Lokalize. DESIGN aims at people who are familiar with KDE/Qt hacking and want to make changes to my program. PACKAGING is of course for packagers to set proper dependencies for the distro package.
Other projects providing a good example include Okular (nice graphical presentation) and plasma, which has it's documents in techbase.
It would be great if other projects had such design overview documents too. So, if you're the author of a project, or you're starting hacking on some app and have just found out smth about design which would save time for future hackers, please update/create design documents and share your knowledge.